Apply suggestions from code review

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

mailserver.env

@@ -134,9 +134,9 @@ ENABLE_IMAP=1
 # **0** => Disabled
 ENABLE_CLAMAV=0
 
-# Add the value as prefix to the mail subject when spam is detected
+# Add the value as a prefix to the mail subject when spam is detected.
 # NOTE: By default spam is delivered to a junk folder, reducing the value of a subject prefix for spam.
-# NOTE: If not using Docker Compose, other CRI may require the single quotes removed.
+# NOTE: When not using Docker Compose, other CRI may not support quote-wrapping the value here to preserve any trailing white-space.
 SPAM_SUBJECT=
 
 # Enables Rspamd

test/tests/parallel/set1/spam_virus/amavis.bats

@@ -74,6 +74,8 @@ function teardown_file() {
   assert_success
   assert_output --partial '$sa_kill_level_deflt = 3.0;'
 
+  # This feature is handled by our SPAM_SUBJECT ENV through a sieve script instead.
+  # Thus the feature here should always be disabled via the 'undef' value.
   _run_in_container grep 'sa_spam_subject_tag' "${AMAVIS_DEFAULTS_FILE}"
   assert_success
   assert_output --partial '$sa_spam_subject_tag = undef;'