Apply suggestions from code review

test/helper/log_and_filtering.bash

@@ -94,6 +94,7 @@ function _print_mail_log_of_queue_id_from_msgid() {
   run echo "${QUEUE_ID}"
   assert_line --regexp "^${QUEUE_ID_REGEX}$"
 
+  # Postfix specific logs:
   _filter_service_log 'mail' "${QUEUE_ID}"
 }
 
@@ -104,5 +105,6 @@ function _print_mail_log_for_msgid() {
   local MSG_ID=$(__construct_msgid "${1:?The local-part for MSG_ID was not provided}")
   shift 1
 
+  # Dovecot specific logs:
   _filter_service_log 'mail' "msgid=${MSG_ID}"
 }

test/helper/sending.bash

@@ -123,7 +123,7 @@ function _send_email_with_msgid() {
   local MSG_ID=$(__construct_msgid "${1:?The local-part for MSG_ID was not provided}")
   shift 1
 
-  _send_email "${@}" --header "Message-Id: ${MID}"
+  _send_email "${@}" --header "Message-Id: ${MSG_ID}"
 }
 
 # Send a spam e-mail by utilizing GTUBE.

test/tests/parallel/set3/mta/smtp_delivery.bats

@@ -228,7 +228,7 @@ function _successful() {
 
 @test "redirects mail to external aliases" {
   _service_log_should_contain_string 'mail' 'Passed CLEAN {RelayedInbound}'
-  run bash -c "grep '<user@external.tld> -> <external1@otherdomain.tld>' <<< '${output}'"
+  assert_output --partial '<user@external.tld> -> <external1@otherdomain.tld>'
   _should_output_number_of_lines 2
   # assert_output --partial 'external.tld=user@example.test> -> <external1@otherdomain.tld>'
 }

test/tests/serial/mail_pop3.bats

@@ -33,7 +33,8 @@ function teardown_file() { _default_teardown ; }
   assert_success
 }
 
-@test 'Mail log is error-free' {
+# TODO: Remove in favor of a common helper method, as described in vmail-id.bats equivalent test-case
+@test 'Mail log is error free' {
   _service_log_should_not_contain_string 'mail' 'non-null host address bits in'
   _service_log_should_not_contain_string 'mail' ': error:'
 }

test/tests/serial/vmail-id.bats

@@ -58,6 +58,7 @@ function teardown_file() { _default_teardown ; }
   _service_log_should_not_contain_string 'mail' '\(!\)connect'
 
   # Postfix: https://github.com/docker-mailserver/docker-mailserver/pull/2597
+   # Log line match example: https://github.com/docker-mailserver/docker-mailserver/pull/2598#issuecomment-1141176633
   _service_log_should_not_contain_string 'mail' 'using backwards-compatible default setting'
 
   # Postgrey: https://github.com/docker-mailserver/docker-mailserver/pull/612#discussion_r117635774