diff --git a/data/Dockerfiles/backup/Dockerfile b/data/Dockerfiles/backup/Dockerfile
index 6234e725b..7f989c399 100644
--- a/data/Dockerfiles/backup/Dockerfile
+++ b/data/Dockerfiles/backup/Dockerfile
@@ -1,3 +1,3 @@
 FROM debian:bookworm-slim
 
-RUN apt update && apt install pigz -y --no-install-recommends
\ No newline at end of file
+RUN apt update && apt install pigz zstd -y --no-install-recommends
\ No newline at end of file
diff --git a/helper-scripts/backup_and_restore.sh b/helper-scripts/backup_and_restore.sh
index c7615c294..7b75272cf 100755
--- a/helper-scripts/backup_and_restore.sh
+++ b/helper-scripts/backup_and_restore.sh
@@ -110,32 +110,32 @@ function backup() {
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_vmail-vol-1$):/vmail:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_vmail.tar.gz /vmail
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_vmail.tar.zst /vmail
       ;;&
     crypt|all)
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_crypt-vol-1$):/crypt:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_crypt.tar.gz /crypt
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_crypt.tar.zst /crypt
       ;;&
     redis|all)
       docker exec $(docker ps -qf name=redis-mailcow) redis-cli -a ${REDISPASS} --no-auth-warning save
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_redis-vol-1$):/redis:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_redis.tar.gz /redis
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_redis.tar.zst /redis
       ;;&
     rspamd|all)
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_rspamd-vol-1$):/rspamd:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_rspamd.tar.gz /rspamd
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_rspamd.tar.zst /rspamd
       ;;&
     postfix|all)
       docker run --name mailcow-backup --rm \
         -v ${BACKUP_LOCATION}/mailcow-${DATE}:/backup:z \
         -v $(docker volume ls -qf name=^${CMPS_PRJ}_postfix-vol-1$):/postfix:ro,z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="pigz --rsyncable -p ${THREADS}" -Pcvpf /backup/backup_postfix.tar.gz /postfix
+        ${DEBIAN_DOCKER_IMAGE} /bin/tar --warning='no-file-ignored' --use-compress-program="zstd --rsyncable -T${THREADS}" -Pcvpf /backup/backup_postfix.tar.zst /postfix
       ;;&
     mysql|all)
       SQLIMAGE=$(grep -iEo '(mysql|mariadb)\:.+' ${COMPOSE_FILE})
@@ -154,7 +154,7 @@ function backup() {
           ${SQLIMAGE} /bin/sh -c "mariabackup --host mysql --user root --password ${DBROOT} --backup --rsync --target-dir=/backup_mariadb ; \
           mariabackup --prepare --target-dir=/backup_mariadb ; \
           chown -R 999:999 /backup_mariadb ; \
-          /bin/tar --warning='no-file-ignored' --use-compress-program='gzip --rsyncable' -Pcvpf /backup/backup_mariadb.tar.gz /backup_mariadb ;"
+          /bin/tar --warning='no-file-ignored' --use-compress-program='zstd --rsyncable' -Pcvpf /backup/backup_mariadb.tar.zst /backup_mariadb ;"
       fi
       ;;&
     --delete-days)
@@ -170,6 +170,19 @@ function backup() {
   done
 }
 
+function get_archive_info() {
+  local backup_name="$1"
+  local location="$2"
+
+  if [[ -f "${location}/${backup_name}.tar.zst" ]]; then
+    echo "${backup_name}.tar.zst|zstd -d -T${THREADS}"
+  elif [[ -f "${location}/${backup_name}.tar.gz" ]]; then
+    echo "${backup_name}.tar.gz|pigz -d -p ${THREADS}"
+  else
+    echo ""
+  fi
+}
+
 function restore() {
   for bin in docker; do
   if [[ -z $(which ${bin}) ]]; then
@@ -199,10 +212,17 @@ function restore() {
     case "$1" in
     vmail)
       docker stop $(docker ps -qf name=dovecot-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_vmail-vol-1$):/vmail:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_vmail.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_vmail" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for vmail (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_vmail-vol-1$):/vmail:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=dovecot-mailcow)
       echo
       echo "In most cases it is not required to run a full resync, you can run the command printed below at any time after testing wether the restore process broke a mailbox:"
@@ -218,31 +238,50 @@ function restore() {
       ;;
     redis)
       docker stop $(docker ps -qf name=redis-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_redis-vol-1$):/redis:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_redis.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_redis" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for redis (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_redis-vol-1$):/redis:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=redis-mailcow)
       ;;
     crypt)
       docker stop $(docker ps -qf name=dovecot-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_crypt-vol-1$):/crypt:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_crypt.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_crypt" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for crypt (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_crypt-vol-1$):/crypt:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=dovecot-mailcow)
       ;;
     rspamd)
-      if [[ $(find "${RESTORE_LOCATION}" \( -name '*x86*' -o -name '*aarch*' \) -exec basename {} \; | sed 's/^\.//' | sed 's/^\.//') == "" ]]; then
+      ARCHIVE_INFO=$(get_archive_info "backup_rspamd" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for rspamd (searched for .tar.zst and .tar.gz)\e[0m"
+      elif [[ $(find "${RESTORE_LOCATION}" \( -name '*x86*' -o -name '*aarch*' \) -exec basename {} \; | sed 's/^\.//' | sed 's/^\.//') == "" ]]; then
         echo -e "\e[33mCould not find a architecture signature of the loaded backup... Maybe the backup was done before the multiarch update?"
         sleep 2
         echo -e "Continuing anyhow. If rspamd is crashing upon boot try remove the rspamd volume with docker volume rm ${CMPS_PRJ}_rspamd-vol-1 after you've stopped the stack.\e[0m"
         sleep 2
         docker stop $(docker ps -qf name=rspamd-mailcow)
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
         docker run -i --name mailcow-backup --rm \
           -v ${RESTORE_LOCATION}:/backup:z \
           -v $(docker volume ls -qf name=^${CMPS_PRJ}_rspamd-vol-1$):/rspamd:z \
-          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_rspamd.tar.gz
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
         docker start $(docker ps -aqf name=rspamd-mailcow)
       elif [[ $ARCH != $(find "${RESTORE_LOCATION}" \( -name '*x86*' -o -name '*aarch*' \) -exec basename {} \; | sed 's/^\.//' | sed 's/^\.//') ]]; then
         echo -e "\e[31mThe Architecture of the backed up mailcow OS is different then your restoring mailcow OS..."
@@ -250,19 +289,28 @@ function restore() {
         echo -e "Skipping rspamd due to compatibility issues!\e[0m"
       else
         docker stop $(docker ps -qf name=rspamd-mailcow)
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
         docker run -i --name mailcow-backup --rm \
           -v ${RESTORE_LOCATION}:/backup:z \
           -v $(docker volume ls -qf name=^${CMPS_PRJ}_rspamd-vol-1$):/rspamd:z \
-          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_rspamd.tar.gz
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
         docker start $(docker ps -aqf name=rspamd-mailcow)
       fi
       ;;
     postfix)
       docker stop $(docker ps -qf name=postfix-mailcow)
-      docker run -i --name mailcow-backup --rm \
-        -v ${RESTORE_LOCATION}:/backup:z \
-        -v $(docker volume ls -qf name=^${CMPS_PRJ}_postfix-vol-1$):/postfix:z \
-        ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -Pxvf /backup/backup_postfix.tar.gz
+      ARCHIVE_INFO=$(get_archive_info "backup_postfix" "${RESTORE_LOCATION}")
+      if [[ -z "${ARCHIVE_INFO}" ]]; then
+        echo -e "\e[31mError: No backup file found for postfix (searched for .tar.zst and .tar.gz)\e[0m"
+      else
+        ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+        DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+        docker run -i --name mailcow-backup --rm \
+          -v ${RESTORE_LOCATION}:/backup:z \
+          -v $(docker volume ls -qf name=^${CMPS_PRJ}_postfix-vol-1$):/postfix:z \
+          ${DEBIAN_DOCKER_IMAGE} /bin/tar --use-compress-program="${DECOMPRESS_PROG}" -Pxvf /backup/${ARCHIVE_FILE}
+      fi
       docker start $(docker ps -aqf name=postfix-mailcow)
       ;;
     mysql|mariadb)
@@ -305,14 +353,19 @@ function restore() {
           echo Restoring... && \
           gunzip < backup/backup_mysql.gz | mysql -uroot && \
           mysql -uroot -e SHUTDOWN;"
-        elif [[ -f "${RESTORE_LOCATION}/backup_mariadb.tar.gz" ]]; then
-        docker run --name mailcow-backup --rm \
-          -v $(docker volume ls -qf name=^${CMPS_PRJ}_mysql-vol-1$):/backup_mariadb/:rw,z \
-          --entrypoint= \
-          -v ${RESTORE_LOCATION}:/backup:z \
-          ${SQLIMAGE} /bin/bash -c "shopt -s dotglob ; \
-            /bin/rm -rf /backup_mariadb/* ; \
-            /bin/tar -Pxvzf /backup/backup_mariadb.tar.gz"
+        else
+          ARCHIVE_INFO=$(get_archive_info "backup_mariadb" "${RESTORE_LOCATION}")
+          if [[ -n "${ARCHIVE_INFO}" ]]; then
+            ARCHIVE_FILE=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f1)
+            DECOMPRESS_PROG=$(echo "${ARCHIVE_INFO}" | cut -d'|' -f2)
+            docker run --name mailcow-backup --rm \
+              -v $(docker volume ls -qf name=^${CMPS_PRJ}_mysql-vol-1$):/backup_mariadb/:rw,z \
+              --entrypoint= \
+              -v ${RESTORE_LOCATION}:/backup:z \
+              ${SQLIMAGE} /bin/bash -c "shopt -s dotglob ; \
+                /bin/rm -rf /backup_mariadb/* ; \
+                /bin/tar --use-compress-program='${DECOMPRESS_PROG}' -Pxvf /backup/${ARCHIVE_FILE}"
+          fi
         fi
         echo "Modifying mailcow.conf..."
         source ${RESTORE_LOCATION}/mailcow.conf
@@ -363,8 +416,8 @@ elif [[ ${1} == "restore" ]]; then
   fi
 
   echo "[ 0 ] - all"
-  # find all files in folder with *.gz extension, print their base names, remove backup_, remove .tar (if present), remove .gz
-  FILE_SELECTION[0]=$(find "${FOLDER_SELECTION[${input_sel}]}" -maxdepth 1 \( -type d -o -type f \) \( -name '*.gz' -o -name 'mysql' \) -printf '%f\n' | sed 's/backup_*//' | sed 's/\.[^.]*$//' | sed 's/\.[^.]*$//')
+  # find all files in folder with *.zst or *.gz extension, print their base names, remove backup_, remove .tar (if present), remove .zst/.gz
+  FILE_SELECTION[0]=$(find "${FOLDER_SELECTION[${input_sel}]}" -maxdepth 1 \( -type d -o -type f \) \( -name '*.zst' -o -name '*.gz' -o -name 'mysql' \) -printf '%f\n' | sed 's/backup_*//' | sed 's/\.[^.]*$//' | sed 's/\.[^.]*$//' | sort -u)
   for file in $(ls -f "${FOLDER_SELECTION[${input_sel}]}"); do
     if [[ ${file} =~ vmail ]]; then
       echo "[ ${i} ] - Mail directory (/var/vmail)"
diff --git a/helper-scripts/dev_tests/test_backup_and_restore.sh b/helper-scripts/dev_tests/test_backup_and_restore.sh
new file mode 100755
index 000000000..3e39d0dbf
--- /dev/null
+++ b/helper-scripts/dev_tests/test_backup_and_restore.sh
@@ -0,0 +1,301 @@
+#!/usr/bin/env bash
+
+# Test script for backup_and_restore.sh
+# Tests backward compatibility with .tar.gz and new .tar.zst format
+
+set -e
+
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+BACKUP_IMAGE="${BACKUP_IMAGE:-ghcr.io/mailcow/backup:latest}"
+TEST_DIR="/tmp/mailcow_backup_test_$$"
+THREADS=2
+
+echo "=== Mailcow Backup & Restore Test Suite ==="
+echo "Test directory: ${TEST_DIR}"
+echo "Backup image: ${BACKUP_IMAGE}"
+echo ""
+
+# Cleanup function
+cleanup() {
+  echo "Cleaning up test files..."
+  rm -rf "${TEST_DIR}"
+  docker rmi mailcow-backup-test 2>/dev/null || true
+}
+trap cleanup EXIT
+
+# Create test directory structure
+mkdir -p "${TEST_DIR}"/{test_data,backup_zst,backup_gz,restore_zst,restore_gz,backup_large_zst,backup_large_gz}
+echo "Test data for mailcow backup compatibility test" > "${TEST_DIR}/test_data/test.txt"
+echo "Additional file to verify complete restore" > "${TEST_DIR}/test_data/test2.txt"
+
+# Build test backup image with zstd support
+echo "=== Building backup image with zstd support ==="
+docker build -t mailcow-backup-test "${SCRIPT_DIR}/../data/Dockerfiles/backup/" || {
+  echo "ERROR: Failed to build backup image"
+  exit 1
+}
+
+# Test 1: Create .tar.zst backup
+echo ""
+echo "=== Test 1: Creating .tar.zst backup ==="
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/test_data:/data:ro" \
+  -v "${TEST_DIR}/backup_zst:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="zstd --rsyncable -T${THREADS}" \
+  -cvpf /backup/backup_test.tar.zst . \
+  > /dev/null
+echo "✓ .tar.zst backup created: $(ls -lh ${TEST_DIR}/backup_zst/backup_test.tar.zst | awk '{print $5}')"
+
+# Test 2: Create .tar.gz backup
+echo ""
+echo "=== Test 2: Creating .tar.gz backup (legacy) ==="
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/test_data:/data:ro" \
+  -v "${TEST_DIR}/backup_gz:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="pigz --rsyncable -p ${THREADS}" \
+  -cvpf /backup/backup_test.tar.gz . \
+  > /dev/null
+echo "✓ .tar.gz backup created: $(ls -lh ${TEST_DIR}/backup_gz/backup_test.tar.gz | awk '{print $5}')"
+
+# Test 3: Test get_archive_info function
+echo ""
+echo "=== Test 3: Testing get_archive_info function ==="
+
+# Extract and test the function directly
+get_archive_info() {
+  local backup_name="$1"
+  local location="$2"
+
+  if [[ -f "${location}/${backup_name}.tar.zst" ]]; then
+    echo "${backup_name}.tar.zst|zstd -d -T${THREADS}"
+  elif [[ -f "${location}/${backup_name}.tar.gz" ]]; then
+    echo "${backup_name}.tar.gz|pigz -d -p ${THREADS}"
+  else
+    echo ""
+  fi
+}
+
+# Test with .tar.zst
+result=$(get_archive_info "backup_test" "${TEST_DIR}/backup_zst")
+if [[ "${result}" =~ "zstd" ]]; then
+  echo "✓ Correctly detects .tar.zst and returns zstd decompressor"
+else
+  echo "✗ Failed to detect .tar.zst"
+  exit 1
+fi
+
+# Test with .tar.gz
+result=$(get_archive_info "backup_test" "${TEST_DIR}/backup_gz")
+if [[ "${result}" =~ "pigz" ]]; then
+  echo "✓ Correctly detects .tar.gz and returns pigz decompressor"
+else
+  echo "✗ Failed to detect .tar.gz"
+  exit 1
+fi
+
+# Test with no file
+result=$(get_archive_info "backup_test" "${TEST_DIR}")
+if [[ -z "${result}" ]]; then
+  echo "✓ Correctly returns empty when no backup file found"
+else
+  echo "✗ Should return empty but got: ${result}"
+  exit 1
+fi
+
+# Test 4: Restore from .tar.zst
+echo ""
+echo "=== Test 4: Restoring from .tar.zst ==="
+docker run --rm \
+  -w /restore \
+  -v "${TEST_DIR}/backup_zst:/backup:ro" \
+  -v "${TEST_DIR}/restore_zst:/restore" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="zstd -d -T${THREADS}" -xvpf /backup/backup_test.tar.zst \
+  > /dev/null 2>&1
+
+if [[ -f "${TEST_DIR}/restore_zst/test.txt" ]] && \
+   [[ -f "${TEST_DIR}/restore_zst/test2.txt" ]]; then
+  echo "✓ Successfully restored from .tar.zst"
+else
+  echo "✗ Failed to restore from .tar.zst"
+  ls -la "${TEST_DIR}/restore_zst/" || true
+  exit 1
+fi
+
+# Test 5: Restore from .tar.gz
+echo ""
+echo "=== Test 5: Restoring from .tar.gz (backward compatibility) ==="
+docker run --rm \
+  -w /restore \
+  -v "${TEST_DIR}/backup_gz:/backup:ro" \
+  -v "${TEST_DIR}/restore_gz:/restore" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="pigz -d -p ${THREADS}" -xvpf /backup/backup_test.tar.gz \
+  > /dev/null 2>&1
+
+if [[ -f "${TEST_DIR}/restore_gz/test.txt" ]] && \
+   [[ -f "${TEST_DIR}/restore_gz/test2.txt" ]]; then
+  echo "✓ Successfully restored from .tar.gz (backward compatible)"
+else
+  echo "✗ Failed to restore from .tar.gz"
+  ls -la "${TEST_DIR}/restore_gz/" || true
+  exit 1
+fi
+
+# Test 6: Verify content integrity
+echo ""
+echo "=== Test 6: Verifying content integrity ==="
+original_content=$(cat "${TEST_DIR}/test_data/test.txt")
+zst_content=$(cat "${TEST_DIR}/restore_zst/test.txt")
+gz_content=$(cat "${TEST_DIR}/restore_gz/test.txt")
+
+if [[ "${original_content}" == "${zst_content}" ]] && \
+   [[ "${original_content}" == "${gz_content}" ]]; then
+  echo "✓ Content integrity verified for both formats"
+else
+  echo "✗ Content mismatch detected"
+  exit 1
+fi
+
+# Test 7: Compare compression ratios
+echo ""
+echo "=== Test 7: Compression comparison ==="
+zst_size=$(stat -f%z "${TEST_DIR}/backup_zst/backup_test.tar.zst" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_zst/backup_test.tar.zst")
+gz_size=$(stat -f%z "${TEST_DIR}/backup_gz/backup_test.tar.gz" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_gz/backup_test.tar.gz")
+improvement=$(echo "scale=2; (${gz_size} - ${zst_size}) * 100 / ${gz_size}" | bc)
+
+echo "  Small files - .tar.gz size: ${gz_size} bytes"
+echo "  Small files - .tar.zst size: ${zst_size} bytes"
+echo "  Small files - Improvement: ${improvement}% smaller with zstd"
+
+# Test 8: Error handling - missing backup file
+echo ""
+echo "=== Test 8: Error handling - Missing backup file ==="
+result=$(get_archive_info "nonexistent_backup" "${TEST_DIR}/backup_zst")
+if [[ -z "${result}" ]]; then
+  echo "✓ Correctly handles missing backup files"
+else
+  echo "✗ Should return empty for missing files"
+  exit 1
+fi
+
+# Test 9: Error handling - Empty directory
+echo ""
+echo "=== Test 9: Error handling - Empty directory ==="
+mkdir -p "${TEST_DIR}/empty_dir"
+result=$(get_archive_info "backup_test" "${TEST_DIR}/empty_dir")
+if [[ -z "${result}" ]]; then
+  echo "✓ Correctly handles empty directories"
+else
+  echo "✗ Should return empty for empty directories"
+  exit 1
+fi
+
+# Test 10: Priority test - .tar.zst preferred over .tar.gz
+echo ""
+echo "=== Test 10: Format priority - .tar.zst preferred ==="
+mkdir -p "${TEST_DIR}/both_formats"
+touch "${TEST_DIR}/both_formats/backup_test.tar.gz"
+touch "${TEST_DIR}/both_formats/backup_test.tar.zst"
+result=$(get_archive_info "backup_test" "${TEST_DIR}/both_formats")
+if [[ "${result}" =~ "zstd" ]]; then
+  echo "✓ Correctly prefers .tar.zst when both formats exist"
+else
+  echo "✗ Should prefer .tar.zst over .tar.gz"
+  exit 1
+fi
+
+# Test 11: Large file compression test
+echo ""
+echo "=== Test 11: Large file compression test ==="
+mkdir -p "${TEST_DIR}/large_data"
+# Create ~10MB of compressible data (log-like content)
+for i in {1..50000}; do
+  echo "[$(date '+%Y-%m-%d %H:%M:%S')] INFO: Processing email message $i from user@example.com to recipient@domain.com" >> "${TEST_DIR}/large_data/maillog.txt"
+  echo "[$(date '+%Y-%m-%d %H:%M:%S')] DEBUG: SMTP connection established from 192.168.1.$((i % 255))" >> "${TEST_DIR}/large_data/maillog.txt"
+done 2>/dev/null
+
+# Get size (portable: works on Linux and macOS)
+if du --version 2>/dev/null | grep -q GNU; then
+  original_size=$(du -sb "${TEST_DIR}/large_data" | cut -f1)
+else
+  # macOS
+  original_size=$(find "${TEST_DIR}/large_data" -type f -exec stat -f%z {} \; | awk '{sum+=$1} END {print sum}')
+fi
+echo "  Original data size: $(echo "scale=2; ${original_size} / 1024 / 1024" | bc) MB"
+
+# Backup with zstd
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/large_data:/data:ro" \
+  -v "${TEST_DIR}/backup_large_zst:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="zstd --rsyncable -T${THREADS}" \
+  -cvpf /backup/backup_large.tar.zst . \
+  > /dev/null 2>&1
+
+# Backup with pigz
+docker run --rm \
+  -w /data \
+  -v "${TEST_DIR}/large_data:/data:ro" \
+  -v "${TEST_DIR}/backup_large_gz:/backup" \
+  mailcow-backup-test \
+  /bin/tar --use-compress-program="pigz --rsyncable -p ${THREADS}" \
+  -cvpf /backup/backup_large.tar.gz . \
+  > /dev/null 2>&1
+
+zst_large_size=$(stat -f%z "${TEST_DIR}/backup_large_zst/backup_large.tar.zst" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_large_zst/backup_large.tar.zst" 2>/dev/null || echo "0")
+gz_large_size=$(stat -f%z "${TEST_DIR}/backup_large_gz/backup_large.tar.gz" 2>/dev/null || stat -c%s "${TEST_DIR}/backup_large_gz/backup_large.tar.gz" 2>/dev/null || echo "0")
+
+if [[ ${zst_large_size} -gt 0 ]] && [[ ${gz_large_size} -gt 0 ]]; then
+  large_improvement=$(echo "scale=2; (${gz_large_size} - ${zst_large_size}) * 100 / ${gz_large_size}" | bc)
+
+  echo "  .tar.gz compressed: $(echo "scale=2; ${gz_large_size} / 1024 / 1024" | bc) MB"
+  echo "  .tar.zst compressed: $(echo "scale=2; ${zst_large_size} / 1024 / 1024" | bc) MB"
+  echo "  Improvement: ${large_improvement}% smaller with zstd"
+else
+  echo "  ✗ Failed to get file sizes"
+  exit 1
+fi
+
+if [[ $(echo "${large_improvement} > 0" | bc) -eq 1 ]]; then
+  echo "✓ zstd provides better compression on realistic data"
+else
+  echo "⚠ zstd compression similar or worse than gzip (unusual but not critical)"
+fi
+
+# Test 12: Thread scaling test
+echo ""
+echo "=== Test 12: Multi-threading verification ==="
+# This test verifies that different thread counts work (not measuring speed difference)
+for thread_count in 1 4; do
+  THREADS=${thread_count}
+  result=$(get_archive_info "backup_test" "${TEST_DIR}/backup_zst")
+  if [[ "${result}" =~ "-T${thread_count}" ]]; then
+    echo "✓ Thread count ${thread_count} correctly configured"
+  else
+    echo "✗ Thread count not properly applied"
+    exit 1
+  fi
+done
+
+echo ""
+echo "=== All tests passed! ==="
+echo ""
+echo "Summary:"
+echo "  ✓ zstd compression working"
+echo "  ✓ pigz compression working (legacy)"
+echo "  ✓ zstd decompression working"
+echo "  ✓ pigz decompression working (backward compatible)"
+echo "  ✓ Archive detection working"
+echo "  ✓ Content integrity verified"
+echo "  ✓ Format priority correct (.tar.zst preferred)"
+echo "  ✓ Error handling for missing files"
+echo "  ✓ Error handling for empty directories"
+echo "  ✓ Multi-threading configuration verified"
+echo "  ✓ Large file compression: ${large_improvement}% improvement"
+echo "  ✓ Small file compression: ${improvement}% improvement"