diff --git a/test/tests/parallel/set3/scripts/postconf-helper.bats b/test/tests/parallel/set3/scripts/postconf-helper.bats
new file mode 100644
index 00000000..d77d50ac
--- /dev/null
+++ b/test/tests/parallel/set3/scripts/postconf-helper.bats
@@ -0,0 +1,48 @@
+load "${REPOSITORY_ROOT}/test/helper/common"
+load "${REPOSITORY_ROOT}/test/helper/setup"
+
+BATS_TEST_NAME_PREFIX='[Scripts] (helper functions) (postfix - _add_to_or_update_postfix_main) '
+CONTAINER_NAME='dms-test_postconf-helper'
+# Various tests for the helper function `_add_to_or_update_postfix_main()`
+function setup_file() {
+  _init_with_defaults
+  _common_container_setup
+  # Begin tests without 'relayhost' defined in 'main.cf'
+  _run_in_container postconf -X relayhost
+  assert_success
+}
+function teardown_file() { _default_teardown ; }
+# Add or modify in Postfix config `main.cf` a parameter key with the provided value.
+# When the key already exists, the new value is appended (default), or prepended (explicitly requested).
+# NOTE: This test-case helper is hard-coded for testing with the 'relayhost' parameter.
+#
+# @param ${1} = new value (appended or prepended)
+# @param ${2} = action "append" (default) or "prepend" [OPTIONAL]
+function _modify_postfix_main_config() {
+  _run_in_container_bash "source /usr/local/bin/helpers/{postfix,utils}.sh && _add_to_or_update_postfix_main relayhost '${1}' '${2}'"
+  _run_in_container grep '^relayhost' '/etc/postfix/main.cf'
+}
+@test "check if initial value is empty" {
+  _run_in_container postconf -h 'relayhost'
+  assert_output ''
+}
+@test "add single value" {
+  _modify_postfix_main_config 'single-value-test'
+  assert_output 'relayhost = single-value-test'
+}
+@test "prepend value" {
+  _modify_postfix_main_config 'prepend-test' 'prepend'
+  assert_output 'relayhost = prepend-test single-value-test'
+}
+@test "append value (explicit)" {
+  _modify_postfix_main_config 'append-test-explicit' 'append'
+  assert_output 'relayhost = prepend-test single-value-test append-test-explicit'
+}
+@test "append value (implicit)" {
+  _modify_postfix_main_config 'append-test-implicit'
+  assert_output 'relayhost = prepend-test single-value-test append-test-explicit append-test-implicit'
+}
+@test "try to append already existing value" {
+  _modify_postfix_main_config 'append-test-implicit'
+  assert_output 'relayhost = prepend-test single-value-test append-test-explicit append-test-implicit'
+}