diff --git a/Makefile b/Makefile
index f717e39e..c38731d4 100644
--- a/Makefile
+++ b/Makefile
@@ -25,7 +25,7 @@ clean:
 # remove running and stopped test containers
 	-@ [[ -d config.bak ]] && { rm -rf config ; mv config.bak config ; } || :
 	-@ [[ -d testconfig.bak ]] && { sudo rm -rf test/config ; mv testconfig.bak test/config ; } || :
-	-@ for container in $$(docker ps -a --filter name='^/mail$$|^ldap_for_mail$$|^mail_override_hostname$$|^open-dkim$$|^hadolint$$|^eclint$$|^shellcheck$$' | sed 1d | cut -f 1-1 -d ' '); do docker rm -f $$container; done
+	-@ for container in $$(docker ps -a --filter name='^/mail$$|^ldap_for_mail$$|^mail_override_hostname$$|^mail_non_subdomain_hostname$$|^open-dkim$$|^hadolint$$|^eclint$$|^shellcheck$$' | sed 1d | cut -f 1-1 -d ' '); do docker rm -f $$container; done
 	-@ sudo rm -rf test/onedir test/alias test/quota test/relay test/config/dovecot-lmtp/userdb test/config/key* test/config/opendkim/keys/domain.tld/ test/config/opendkim/keys/example.com/ test/config/opendkim/keys/localdomain2.com/ test/config/postfix-aliases.cf test/config/postfix-receive-access.cf test/config/postfix-receive-access.cfe test/config/dovecot-quotas.cf test/config/postfix-send-access.cf test/config/postfix-send-access.cfe test/config/relay-hosts/chksum test/config/relay-hosts/postfix-aliases.cf test/config/dhparams.pem test/config/dovecot-lmtp/dh.pem test/config/relay-hosts/dovecot-quotas.cf test/config/user-patches.sh test/alias/config/postfix-virtual.cf test/quota/config/dovecot-quotas.cf test/quota/config/postfix-accounts.cf test/relay/config/postfix-relaymap.cf test/relay/config/postfix-sasl-password.cf test/duplicate_configs/
 
 # -----------------------------------------------
diff --git a/docs/content/config/environment.md b/docs/content/config/environment.md
index e134648b..923d3dd2 100644
--- a/docs/content/config/environment.md
+++ b/docs/content/config/environment.md
@@ -11,7 +11,7 @@ title: Environment Variables
 ##### OVERRIDE_HOSTNAME
 
 - empty => uses the `hostname` command to get the mail server's canonical hostname.
-- => Specify a fully-qualified domainname to serve mail for.  This is used for many of the config features so if you can't set your hostname (e.g. you're in a container platform that doesn't let you) specify it in this environment variable.
+- => Specify a fully-qualified domainname to serve mail for.  This is used for many of the config features so if you can't set your hostname (e.g. you're in a container platform that doesn't let you) specify it in this environment variable. It will take priority over your docker-compose.yml's `hostname:` and `domainname:` values.
 
 ##### DMS_DEBUG
 
diff --git a/target/scripts/check-for-changes.sh b/target/scripts/check-for-changes.sh
index f089a5cc..7223d729 100755
--- a/target/scripts/check-for-changes.sh
+++ b/target/scripts/check-for-changes.sh
@@ -30,12 +30,7 @@ fi
 
 # determine postmaster address, duplicated from start-mailserver.sh
 # this script previously didn't work when POSTMASTER_ADDRESS was empty
-if [[ -n ${OVERRIDE_HOSTNAME} ]]
-then
-  DOMAINNAME="${OVERRIDE_HOSTNAME#*.}"
-else
-  DOMAINNAME="$(hostname -d)"
-fi
+_obtain_hostname_and_domainname
 
 PM_ADDRESS="${POSTMASTER_ADDRESS:=postmaster@${DOMAINNAME}}"
 _notify 'inf' "${LOG_DATE} Using postmaster address ${PM_ADDRESS}"
diff --git a/target/scripts/helper-functions.sh b/target/scripts/helper-functions.sh
index d94091ff..5967e82a 100755
--- a/target/scripts/helper-functions.sh
+++ b/target/scripts/helper-functions.sh
@@ -209,6 +209,31 @@ function _monitored_files_checksums
 }
 export -f _monitored_files_checksums
 
+# ? --------------------------------------------- General
+
+function _obtain_hostname_and_domainname
+{
+  if [[ -n "${OVERRIDE_HOSTNAME}" ]]
+  then
+    export HOSTNAME="${OVERRIDE_HOSTNAME}"
+    export DOMAINNAME="${DOMAINNAME:-${HOSTNAME#*.}}"
+    # Handle situations where the hostname is name.tld and hostname -d ends up just showing "tld"
+    if [[ ! "${DOMAINNAME}" =~ .*\..* ]]
+    then
+      DOMAINNAME="${HOSTNAME}"
+    fi
+  else
+    # These hostname commands will fail with "hostname: Name or service not known"
+    # if the hostname is not valid (important for tests)
+    HOSTNAME="$(hostname -f)"
+    DOMAINNAME="${DOMAINNAME:-$(hostname -d)}"
+    if [[ ! "${DOMAINNAME}" =~ .*\..* ]]
+    then
+      DOMAINNAME="${HOSTNAME}"
+    fi
+  fi
+}
+
 function _shutdown
 {
   _notify 'err' "Shutting down.."
diff --git a/target/scripts/postsrsd-wrapper.sh b/target/scripts/postsrsd-wrapper.sh
index 0ba1ba8f..1ad3db23 100755
--- a/target/scripts/postsrsd-wrapper.sh
+++ b/target/scripts/postsrsd-wrapper.sh
@@ -1,18 +1,17 @@
 #! /bin/bash
 
+# shellcheck source=./helper-functions.sh
+. /usr/local/bin/helper-functions.sh
+
 function _generate_secret { ( umask 0077 ; dd if=/dev/urandom bs=24 count=1 2>/dev/null | base64 -w0 > "${1}" ; ) ; }
 
-if [[ -n ${SRS_DOMAINNAME} ]]
+_obtain_hostname_and_domainname
+
+if [[ -n "${SRS_DOMAINNAME}" ]]
 then
   NEW_DOMAIN_NAME="${SRS_DOMAINNAME}"
-elif [[ -n ${OVERRIDE_HOSTNAME} ]]
-then
-  NEW_DOMAIN_NAME="${OVERRIDE_HOSTNAME#*.}"
-elif [[ -n ${DOMAINNAME} ]]
-then
-  NEW_DOMAIN_NAME="${DOMAINNAME}"
 else
-  NEW_DOMAIN_NAME=$(hostname -d)
+  NEW_DOMAIN_NAME="${DOMAINNAME}"
 fi
 
 sed -i -e "s/localdomain/${NEW_DOMAIN_NAME}/g" /etc/default/postsrsd
diff --git a/target/scripts/start-mailserver.sh b/target/scripts/start-mailserver.sh
index 160403ba..7b1e1c5c 100755
--- a/target/scripts/start-mailserver.sh
+++ b/target/scripts/start-mailserver.sh
@@ -67,8 +67,8 @@ VARS[VIRUSMAILS_DELETE_DELAY]="${VIRUSMAILS_DELETE_DELAY:=7}"
 
 export HOSTNAME DOMAINNAME CHKSUM_FILE
 
-HOSTNAME="$(hostname -f)"
-DOMAINNAME="$(hostname -d)"
+_obtain_hostname_and_domainname
+
 CHKSUM_FILE=/tmp/docker-mailserver-config-chksum
 
 # ------------------------------------------------------------
diff --git a/target/scripts/startup/check-stack.sh b/target/scripts/startup/check-stack.sh
index 0f3203e9..f7061d35 100644
--- a/target/scripts/startup/check-stack.sh
+++ b/target/scripts/startup/check-stack.sh
@@ -13,12 +13,6 @@ function _check_hostname
 {
   _notify 'task' 'Checking that hostname/domainname is provided or overridden'
 
-  if [[ -n ${OVERRIDE_HOSTNAME} ]]
-  then
-    export HOSTNAME=${OVERRIDE_HOSTNAME}
-    export DOMAINNAME="${HOSTNAME#*.}"
-  fi
-
   _notify 'inf' "Domain has been set to ${DOMAINNAME}"
   _notify 'inf' "Hostname has been set to ${HOSTNAME}"
 
diff --git a/test/mail_hostname.bats b/test/mail_hostname.bats
new file mode 100644
index 00000000..3d13618e
--- /dev/null
+++ b/test/mail_hostname.bats
@@ -0,0 +1,202 @@
+load 'test_helper/common'
+
+function setup() {
+    run_setup_file_if_necessary
+}
+
+function setup_file() {
+  local PRIVATE_CONFIG
+  PRIVATE_CONFIG="$(duplicate_config_for_container . mail_override_hostname)"
+	docker run --rm -d --name mail_override_hostname \
+		-v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
+		-v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+		-e PERMIT_DOCKER=network \
+		-e DMS_DEBUG=0 \
+		-e ENABLE_SRS=1 \
+		-e OVERRIDE_HOSTNAME=mail.my-domain.com \
+		-h unknown.domain.tld \
+		-t "${NAME}"
+
+  PRIVATE_CONFIG_TWO="$(duplicate_config_for_container . mail_non_subdomain_hostname)"
+	docker run --rm -d --name mail_non_subdomain_hostname \
+		-v "${PRIVATE_CONFIG_TWO}":/tmp/docker-mailserver \
+		-v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+		-e PERMIT_DOCKER=network \
+		-e ENABLE_SRS=1 \
+		-e DMS_DEBUG=0 \
+		--hostname domain.com \
+    --domainname domain.com \
+		-t "${NAME}"
+
+  PRIVATE_CONFIG_THREE="$(duplicate_config_for_container . mail_srs_domainname)"
+  docker run --rm -d --name mail_srs_domainname \
+    -v "${PRIVATE_CONFIG_THREE}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -e PERMIT_DOCKER=network \
+    -e DMS_DEBUG=0 \
+    -e ENABLE_SRS=1 \
+    -e SRS_DOMAINNAME=srs.my-domain.com \
+    -e DOMAINNAME=my-domain.com \
+    -h unknown.domain.tld \
+    -t "${NAME}"
+
+  PRIVATE_CONFIG_FOUR="$(duplicate_config_for_container . mail_domainname)"
+  docker run --rm -d --name mail_domainname \
+    -v "${PRIVATE_CONFIG_FOUR}":/tmp/docker-mailserver \
+    -v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
+    -e PERMIT_DOCKER=network \
+    -e DMS_DEBUG=0 \
+    -e ENABLE_SRS=1 \
+    -e DOMAINNAME=my-domain.com \
+    -h unknown.domain.tld \
+    -t "${NAME}"
+
+  wait_for_smtp_port_in_container mail_override_hostname
+  
+  wait_for_smtp_port_in_container mail_non_subdomain_hostname
+  
+  wait_for_smtp_port_in_container mail_srs_domainname
+
+  wait_for_smtp_port_in_container mail_domainname
+
+  # postfix virtual transport lmtp
+  docker exec mail_override_hostname /bin/sh -c "nc 0.0.0.0 25 < /tmp/docker-mailserver-test/email-templates/existing-user1.txt"
+  docker exec mail_non_subdomain_hostname /bin/sh -c "nc 0.0.0.0 25 < /tmp/docker-mailserver-test/email-templates/existing-user1.txt"
+}
+
+@test "first" {
+  skip 'only used to call setup_file from setup'
+}
+
+@test "checking SRS: SRS_DOMAINNAME is used correctly" {
+  repeat_until_success_or_timeout 15 docker exec mail_srs_domainname grep "SRS_DOMAIN=srs.my-domain.com" /etc/default/postsrsd
+}
+
+@test "checking SRS: DOMAINNAME is handled correctly" {
+  repeat_until_success_or_timeout 15 docker exec mail_domainname grep "SRS_DOMAIN=my-domain.com" /etc/default/postsrsd
+}
+
+@test "checking configuration: hostname/domainname override: check container hostname is applied correctly" {
+  run docker exec mail_override_hostname /bin/bash -c "hostname | grep unknown.domain.tld"
+  assert_success
+}
+
+@test "checking configuration: hostname/domainname override: check overriden hostname is applied to all configs" {
+  run docker exec mail_override_hostname /bin/bash -c "cat /etc/mailname | grep my-domain.com"
+  assert_success
+  run docker exec mail_override_hostname /bin/bash -c "postconf -n | grep mydomain | grep my-domain.com"
+  assert_success
+  run docker exec mail_override_hostname /bin/bash -c "postconf -n | grep myhostname | grep mail.my-domain.com"
+  assert_success
+  run docker exec mail_override_hostname /bin/bash -c "doveconf | grep hostname | grep mail.my-domain.com"
+  assert_success
+  run docker exec mail_override_hostname /bin/bash -c "cat /etc/opendmarc.conf | grep AuthservID | grep mail.my-domain.com"
+  assert_success
+  run docker exec mail_override_hostname /bin/bash -c "cat /etc/opendmarc.conf | grep TrustedAuthservIDs | grep mail.my-domain.com"
+  assert_success
+  run docker exec mail_override_hostname /bin/bash -c "cat /etc/amavis/conf.d/05-node_id | grep myhostname | grep mail.my-domain.com"
+  assert_success
+}
+
+@test "checking configuration: hostname/domainname override: check hostname in postfix HELO message" {
+  run docker exec mail_override_hostname /bin/bash -c "nc -w 1 0.0.0.0 25 | grep mail.my-domain.com"
+  assert_success
+}
+
+@test "checking configuration: hostname/domainname override: check headers of received mail" {
+  run docker exec mail_override_hostname /bin/sh -c "ls -A /var/mail/localhost.localdomain/user1/new | wc -l | grep 1"
+  assert_success
+  run docker exec mail_override_hostname /bin/sh -c "cat /var/mail/localhost.localdomain/user1/new/* | grep mail.my-domain.com"
+  assert_success
+
+  # test whether the container hostname is not found in received mail
+  run docker exec mail_override_hostname /bin/sh -c "cat /var/mail/localhost.localdomain/user1/new/* | grep unknown.domain.tld"
+  assert_failure
+}
+
+@test "checking SRS: OVERRIDE_HOSTNAME is handled correctly" {
+  run docker exec mail_override_hostname grep "SRS_DOMAIN=my-domain.com" /etc/default/postsrsd
+  assert_success
+}
+
+@test "checking dovecot: postmaster address" {
+  run docker exec mail_override_hostname /bin/sh -c "grep 'postmaster_address = postmaster@my-domain.com' /etc/dovecot/conf.d/15-lda.conf"
+  assert_success
+}
+
+#
+# non-subdomain tests
+#
+
+@test "checking configuration: non-subdomain: check container hostname is applied correctly" {
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "hostname | grep domain.com"
+  assert_success
+}
+
+@test "checking configuration: non-subdomain: check overriden hostname is applied to all configs" {
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "cat /etc/mailname | grep domain.com"
+  assert_success
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "postconf -n | grep mydomain | grep domain.com"
+  assert_success
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "postconf -n | grep myhostname | grep domain.com"
+  assert_success
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "doveconf | grep hostname | grep domain.com"
+  assert_success
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "cat /etc/opendmarc.conf | grep AuthservID | grep domain.com"
+  assert_success
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "cat /etc/opendmarc.conf | grep TrustedAuthservIDs | grep domain.com"
+  assert_success
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "cat /etc/amavis/conf.d/05-node_id | grep myhostname | grep domain.com"
+  assert_success
+}
+
+@test "checking configuration: non-subdomain: check hostname in postfix HELO message" {
+  run docker exec mail_non_subdomain_hostname /bin/bash -c "nc -w 1 0.0.0.0 25 | grep domain.com"
+  assert_success
+}
+
+@test "checking configuration: non-subdomain: check headers of received mail" {
+  run docker exec mail_non_subdomain_hostname /bin/sh -c "ls -A /var/mail/localhost.localdomain/user1/new | wc -l | grep 1"
+  assert_success
+  run docker exec mail_non_subdomain_hostname /bin/sh -c "cat /var/mail/localhost.localdomain/user1/new/* | grep domain.com"
+  assert_success
+}
+
+@test "checking SRS: non-subdomain is handled correctly" {
+  docker exec mail_non_subdomain_hostname cat /etc/default/postsrsd
+  run docker exec mail_non_subdomain_hostname grep "SRS_DOMAIN=domain.com" /etc/default/postsrsd
+  assert_success
+}
+
+@test "checking dovecot: non-subdomain postmaster address" {
+  run docker exec mail_non_subdomain_hostname /bin/sh -c "grep 'postmaster_address = postmaster@domain.com' /etc/dovecot/conf.d/15-lda.conf"
+  assert_success
+}
+
+#
+# clean exit
+#
+
+@test "checking that the container stops cleanly: mail_override_hostname" {
+  run docker stop -t 60 mail_override_hostname
+  assert_success
+}
+
+@test "checking that the container stops cleanly: mail_non_subdomain_hostname" {
+  run docker stop -t 60 mail_non_subdomain_hostname
+  assert_success
+}
+
+@test "checking that the container stops cleanly: mail_srs_domainname" {
+  run docker stop -t 60 mail_srs_domainname
+  assert_success
+}
+
+@test "checking that the container stops cleanly: mail_domainname" {
+  run docker stop -t 60 mail_domainname
+  assert_success
+}
+
+@test "last" {
+    skip 'only used to call teardown_file from teardown'
+}
diff --git a/test/mail_override_hostname.bats b/test/mail_override_hostname.bats
deleted file mode 100644
index 3eb2406b..00000000
--- a/test/mail_override_hostname.bats
+++ /dev/null
@@ -1,88 +0,0 @@
-load 'test_helper/common'
-
-function setup() {
-    run_setup_file_if_necessary
-}
-
-function setup_file() {
-  local PRIVATE_CONFIG
-  PRIVATE_CONFIG="$(duplicate_config_for_container .)"
-	docker run --rm -d --name mail_override_hostname \
-		-v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
-		-v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
-		-e PERMIT_DOCKER=network \
-		-e DMS_DEBUG=0 \
-		-e ENABLE_SRS=1 \
-		-e OVERRIDE_HOSTNAME=mail.my-domain.com \
-		-h unknown.domain.tld \
-		-t "${NAME}"
-
-    wait_for_smtp_port_in_container mail_override_hostname
-    # postfix virtual transport lmtp
-	docker exec mail_override_hostname /bin/sh -c "nc 0.0.0.0 25 < /tmp/docker-mailserver-test/email-templates/existing-user1.txt"
-}
-
-@test "first" {
-    skip 'only used to call setup_file from setup'
-}
-
-@test "checking configuration: hostname/domainname override: check container hostname is applied correctly" {
-  run docker exec mail_override_hostname /bin/bash -c "hostname | grep unknown.domain.tld"
-  assert_success
-}
-
-@test "checking configuration: hostname/domainname override: check overriden hostname is applied to all configs" {
-  run docker exec mail_override_hostname /bin/bash -c "cat /etc/mailname | grep my-domain.com"
-  assert_success
-  run docker exec mail_override_hostname /bin/bash -c "postconf -n | grep mydomain | grep my-domain.com"
-  assert_success
-  run docker exec mail_override_hostname /bin/bash -c "postconf -n | grep myhostname | grep mail.my-domain.com"
-  assert_success
-  run docker exec mail_override_hostname /bin/bash -c "doveconf | grep hostname | grep mail.my-domain.com"
-  assert_success
-  run docker exec mail_override_hostname /bin/bash -c "cat /etc/opendmarc.conf | grep AuthservID | grep mail.my-domain.com"
-  assert_success
-  run docker exec mail_override_hostname /bin/bash -c "cat /etc/opendmarc.conf | grep TrustedAuthservIDs | grep mail.my-domain.com"
-  assert_success
-  run docker exec mail_override_hostname /bin/bash -c "cat /etc/amavis/conf.d/05-node_id | grep myhostname | grep mail.my-domain.com"
-  assert_success
-}
-
-@test "checking configuration: hostname/domainname override: check hostname in postfix HELO message" {
-  run docker exec mail_override_hostname /bin/bash -c "nc -w 1 0.0.0.0 25 | grep mail.my-domain.com"
-  assert_success
-}
-
-@test "checking configuration: hostname/domainname override: check headers of received mail" {
-  run docker exec mail_override_hostname /bin/sh -c "ls -A /var/mail/localhost.localdomain/user1/new | wc -l | grep 1"
-  assert_success
-  run docker exec mail_override_hostname /bin/sh -c "cat /var/mail/localhost.localdomain/user1/new/* | grep mail.my-domain.com"
-  assert_success
-
-  # test whether the container hostname is not found in received mail
-  run docker exec mail_override_hostname /bin/sh -c "cat /var/mail/localhost.localdomain/user1/new/* | grep unknown.domain.tld"
-  assert_failure
-}
-
-@test "checking SRS: OVERRIDE_HOSTNAME is handled correctly" {
-  run docker exec mail_override_hostname grep "SRS_DOMAIN=my-domain.com" /etc/default/postsrsd
-  assert_success
-}
-
-@test "checking dovecot: postmaster address" {
-  run docker exec mail_override_hostname /bin/sh -c "grep 'postmaster_address = postmaster@my-domain.com' /etc/dovecot/conf.d/15-lda.conf"
-  assert_success
-}
-
-#
-# clean exit
-#
-
-@test "checking that the container stops cleanly" {
-  run docker stop -t 60 mail_override_hostname
-  assert_success
-}
-
-@test "last" {
-    skip 'only used to call teardown_file from teardown'
-}
diff --git a/test/mail_srs_domainname.bats b/test/mail_srs_domainname.bats
deleted file mode 100644
index aa730476..00000000
--- a/test/mail_srs_domainname.bats
+++ /dev/null
@@ -1,38 +0,0 @@
-load 'test_helper/common'
-
-@test "checking SRS: SRS_DOMAINNAME is used correctly" {
-	local PRIVATE_CONFIG
-  	PRIVATE_CONFIG="$(duplicate_config_for_container . mail_srs_domainname)"
-    docker run --rm -d --name mail_srs_domainname \
-				-v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
-				-v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
-				-e PERMIT_DOCKER=network \
-				-e DMS_DEBUG=0 \
-				-e ENABLE_SRS=1 \
-				-e SRS_DOMAINNAME=srs.my-domain.com \
-				-e DOMAINNAME=my-domain.com \
-				-h unknown.domain.tld \
-				-t "${NAME}"
-
-    teardown() { docker rm -f mail_srs_domainname; }
-
-    repeat_until_success_or_timeout 15 docker exec mail_srs_domainname grep "SRS_DOMAIN=srs.my-domain.com" /etc/default/postsrsd
-}
-
-@test "checking SRS: DOMAINNAME is handled correctly" {
-	local PRIVATE_CONFIG
-  	PRIVATE_CONFIG="$(duplicate_config_for_container . mail_domainname)"
-    docker run --rm -d --name mail_domainname \
-				-v "${PRIVATE_CONFIG}":/tmp/docker-mailserver \
-				-v "$(pwd)/test/test-files":/tmp/docker-mailserver-test:ro \
-				-e PERMIT_DOCKER=network \
-				-e DMS_DEBUG=0 \
-				-e ENABLE_SRS=1 \
-				-e DOMAINNAME=my-domain.com \
-				-h unknown.domain.tld \
-				-t "${NAME}"
-
-    teardown() { docker rm -f mail_domainname; }
-
-    repeat_until_success_or_timeout 15 docker exec mail_domainname grep "SRS_DOMAIN=my-domain.com" /etc/default/postsrsd
-}