diff --git a/docs/content/config/environment.md b/docs/content/config/environment.md
index e5aba889..678ec965 100644
--- a/docs/content/config/environment.md
+++ b/docs/content/config/environment.md
@@ -33,6 +33,18 @@ Here you can adjust the [log-level for Supervisor](http://supervisord.org/loggin
 
 The log-level will show everything in its class and above.
 
+##### DMS_VMAIL_UID
+
+Default: 5000
+
+The User ID assigned to the static vmail user for `/var/mail` (_Mail storage managed by Dovecot_).
+
+##### DMS_VMAIL_GID
+
+Default: 5000
+
+The Group ID assigned to the static vmail group for `/var/mail` (_Mail storage managed by Dovecot_).
+
 ##### ONE_DIR
 
 - 0 => state in default directories.
diff --git a/mailserver.env b/mailserver.env
index 632f6edf..1a57ceca 100644
--- a/mailserver.env
+++ b/mailserver.env
@@ -34,6 +34,12 @@ SUPERVISOR_LOGLEVEL=
 # 1 => consolidate all states into a single directory (`/var/mail-state`) to allow persistence using docker volumes
 ONE_DIR=1
 
+# Support for deployment where these defaults are not compatible (eg: some NAS appliances):
+# /var/mail vmail User ID (default: 5000)
+DMS_VMAIL_UID=
+# /var/mail vmail Group ID (default: 5000)
+DMS_VMAIL_GID=
+
 # **empty** => use FILE
 # LDAP => use LDAP authentication
 # OIDC => use OIDC authentication (not yet implemented)
diff --git a/target/scripts/helpers/accounts.sh b/target/scripts/helpers/accounts.sh
index 82b626c1..7499a2ec 100644
--- a/target/scripts/helpers/accounts.sh
+++ b/target/scripts/helpers/accounts.sh
@@ -70,7 +70,7 @@ function _create_accounts() {
 
       # Dovecot's userdb has the following format
       # user:password:uid:gid:(gecos):home:(shell):extra_fields
-      DOVECOT_USERDB_LINE="${LOGIN}:${PASS}:5000:5000::/var/mail/${DOMAIN}/${USER}/home::${USER_ATTRIBUTES}"
+      DOVECOT_USERDB_LINE="${LOGIN}:${PASS}:${DMS_VMAIL_UID}:${DMS_VMAIL_GID}::/var/mail/${DOMAIN}/${USER}/home::${USER_ATTRIBUTES}"
       if grep -qF "${DOVECOT_USERDB_LINE}" "${DOVECOT_USERDB_FILE}"; then
         _log 'warn' "Login '${LOGIN}' will not be added to '${DOVECOT_USERDB_FILE}' twice"
       else
@@ -141,7 +141,7 @@ function _create_dovecot_alias_dummy_accounts() {
         fi
       fi
 
-      DOVECOT_USERDB_LINE="${ALIAS}:${REAL_ACC[1]}:5000:5000::/var/mail/${REAL_DOMAINNAME}/${REAL_USERNAME}::${REAL_ACC[2]:-}"
+      DOVECOT_USERDB_LINE="${ALIAS}:${REAL_ACC[1]}:${DMS_VMAIL_UID}:${DMS_VMAIL_GID}::/var/mail/${REAL_DOMAINNAME}/${REAL_USERNAME}::${REAL_ACC[2]:-}"
       if grep -qi "^${ALIAS}:" "${DOVECOT_USERDB_FILE}"; then
         _log 'warn' "Alias '${ALIAS}' will not be added to '${DOVECOT_USERDB_FILE}' twice"
       else
diff --git a/target/scripts/helpers/utils.sh b/target/scripts/helpers/utils.sh
index c74fd7e8..e44f0aff 100644
--- a/target/scripts/helpers/utils.sh
+++ b/target/scripts/helpers/utils.sh
@@ -39,9 +39,9 @@ function _get_dms_env_value() {
 # /var/mail folders (used during startup and change detection handling).
 function _chown_var_mail_if_necessary() {
   # fix permissions, but skip this if 3 levels deep the user id is already set
-  if find /var/mail -maxdepth 3 -a \( \! -user 5000 -o \! -group 5000 \) | read -r; then
+  if find /var/mail -maxdepth 3 -a \( \! -user "${DMS_VMAIL_UID}" -o \! -group "${DMS_VMAIL_GID}" \) | read -r; then
     _log 'trace' 'Fixing /var/mail permissions'
-    chown -R 5000:5000 /var/mail || return 1
+    chown -R "${DMS_VMAIL_UID}:${DMS_VMAIL_GID}" /var/mail || return 1
   fi
 }
 
diff --git a/target/scripts/start-mailserver.sh b/target/scripts/start-mailserver.sh
index e41ae71e..f0f385f3 100755
--- a/target/scripts/start-mailserver.sh
+++ b/target/scripts/start-mailserver.sh
@@ -39,6 +39,7 @@ function _register_functions() {
 
   # ? >> Setup
 
+  _register_setup_function '_setup_vmail_id'
   _register_setup_function '_setup_logs_general'
   _register_setup_function '_setup_timezone'
 
diff --git a/target/scripts/startup/setup.d/vmail-id.sh b/target/scripts/startup/setup.d/vmail-id.sh
new file mode 100644
index 00000000..2b692169
--- /dev/null
+++ b/target/scripts/startup/setup.d/vmail-id.sh
@@ -0,0 +1,12 @@
+#!/bin/bash
+
+function _setup_vmail_id() {
+  if [[ "${DMS_VMAIL_UID}" != "5000" ]]; then
+    _log 'debug' "Setting 'docker' UID to ${DMS_VMAIL_UID}"
+    usermod --uid "${DMS_VMAIL_UID}" docker
+  fi
+  if [[ "${DMS_VMAIL_GID}" != "5000" ]]; then
+    _log 'debug' "Setting 'docker' GID to ${DMS_VMAIL_GID}"
+    groupmod --gid "${DMS_VMAIL_GID}" docker
+  fi
+}
diff --git a/target/scripts/startup/variables-stack.sh b/target/scripts/startup/variables-stack.sh
index b18a61dc..3b575f50 100644
--- a/target/scripts/startup/variables-stack.sh
+++ b/target/scripts/startup/variables-stack.sh
@@ -46,6 +46,8 @@ function __environment_variables_general_setup() {
   VARS[POSTMASTER_ADDRESS]="${POSTMASTER_ADDRESS:=postmaster@${DOMAINNAME}}"
   VARS[REPORT_RECIPIENT]="${REPORT_RECIPIENT:=${POSTMASTER_ADDRESS}}"
   VARS[REPORT_SENDER]="${REPORT_SENDER:=mailserver-report@${HOSTNAME}}"
+  VARS[DMS_VMAIL_UID]="${DMS_VMAIL_UID:=5000}"
+  VARS[DMS_VMAIL_GID]="${DMS_VMAIL_GID:=5000}"
 
   _log 'trace' 'Setting anti-spam & anti-virus environment variables'
 
diff --git a/test/tests/serial/vmail-id.bats b/test/tests/serial/vmail-id.bats
new file mode 100644
index 00000000..b44670b2
--- /dev/null
+++ b/test/tests/serial/vmail-id.bats
@@ -0,0 +1,75 @@
+load "${REPOSITORY_ROOT}/test/helper/common"
+load "${REPOSITORY_ROOT}/test/helper/setup"
+
+BATS_TEST_NAME_PREFIX='[ENV] (DMS_VMAIL_UID + DMS_VMAIL_GID) '
+CONTAINER_NAME='dms-test_env-change-vmail-id'
+
+function setup_file() {
+  _init_with_defaults
+
+  local CUSTOM_SETUP_ARGUMENTS=(
+    --env PERMIT_DOCKER=container
+    --env DMS_VMAIL_UID=9042
+    --env DMS_VMAIL_GID=9042
+  )
+
+  _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
+  _wait_for_smtp_port_in_container
+}
+
+function teardown_file() { _default_teardown ; }
+
+@test 'should successfully deliver mail' {
+  _send_email 'email-templates/existing-user1'
+  _wait_for_empty_mail_queue_in_container
+
+  # Should be successfully sent (received) by Postfix:
+  _run_in_container grep 'to=<user1@localhost.localdomain>' /var/log/mail/mail.log
+  assert_success
+  assert_output --partial 'status=sent'
+  _should_output_number_of_lines 1
+
+  # Verify successful delivery via Dovecot to `/var/mail` account by searching for the subject:
+  _repeat_in_container_until_success_or_timeout 20 "${CONTAINER_NAME}" grep -R \
+    'Subject: Test Message existing-user1.txt' \
+    '/var/mail/localhost.localdomain/user1/new/'
+  assert_success
+  _should_output_number_of_lines 1
+}
+
+# TODO: Migrate to test/helper/common.bash
+# This test case is shared with tests.bats, but provides context on errors + some minor edits
+# TODO: Could improve in future with keywords from https://github.com/docker-mailserver/docker-mailserver/pull/3550#issuecomment-1738509088
+# Potentially via a helper that allows an optional fixed number of errors to be present if they were intentional
+@test '/var/log/mail/mail.log is error free' {
+  # Postfix: https://serverfault.com/questions/934703/postfix-451-4-3-0-temporary-lookup-failure
+  _run_in_container grep 'non-null host address bits in' /var/log/mail/mail.log
+  assert_failure
+
+  # Postfix delivery failure: https://github.com/docker-mailserver/docker-mailserver/issues/230
+  _run_in_container grep 'mail system configuration error' /var/log/mail/mail.log
+  assert_failure
+
+  # Unknown error source: https://github.com/docker-mailserver/docker-mailserver/pull/85
+  _run_in_container grep -i ': error:' /var/log/mail/mail.log
+  assert_failure
+
+  # Unknown error source: https://github.com/docker-mailserver/docker-mailserver/pull/320
+  _run_in_container grep -i 'not writable' /var/log/mail/mail.log
+  assert_failure
+  _run_in_container grep -i 'permission denied' /var/log/mail/mail.log
+  assert_failure
+
+  # Amavis: https://forum.howtoforge.com/threads/postfix-smtp-error-caused-by-clamav-cant-connect-to-a-unix-socket-var-run-clamav-clamd-ctl.81002/
+  _run_in_container grep -i '(!)connect' /var/log/mail/mail.log
+  assert_failure
+
+  # Postfix: https://github.com/docker-mailserver/docker-mailserver/pull/2597
+  _run_in_container grep -i 'using backwards-compatible default setting' /var/log/mail/mail.log
+  assert_failure
+
+  # Postgrey: https://github.com/docker-mailserver/docker-mailserver/pull/612#discussion_r117635774
+  _run_in_container grep -i 'connect to 127.0.0.1:10023: Connection refused' /var/log/mail/mail.log
+  assert_failure
+}
+