diff --git a/docs/content/config/troubleshooting/faq.md b/docs/content/config/troubleshooting/faq.md
index 2d6e3ee2..720b2108 100644
--- a/docs/content/config/troubleshooting/faq.md
+++ b/docs/content/config/troubleshooting/faq.md
@@ -51,6 +51,17 @@ Please use `STARTTLS`.
 ### How can I manage my custom Spamassassin rules?
 Antispam rules are managed in `config/spamassassin-rules.cf`.
 
+### What are acceptable `SA_SPAM_SUBJECT` values?
+
+For no subject set `SA_SPAM_SUBJECT=undef`.
+
+For a trailing white-space subject one can define the whole variable with quotes in `docker-compose.yml`:
+
+```docker-compose
+    environment:
+      - "SA_SPAM_SUBJECT=[SPAM] "
+```
+
 ### Why are Spamassassin x-headers not inserted into my sample.domain.com subdomain emails?
 
 In the default setup, amavis only applies Spamassassin x-headers into domains matching the template listed in the config  file 05-domain_id (in the amavis defaults). The default setup @local_domains_acl = ( ".$mydomain" ); does not match subdomains. To match subdomains, you can override the @local_domains_acl directive in the amavis user config file 50-user with @local_domains_maps = ("."); to match any sort of domain template.