Add attachment size info

docs/u_e-postfix-attachment_size.md

@@ -0,0 +1 @@
+Open `data/conf/postfix/main.cf` and set the `message_size_limit` accordingly in bytes.

mkdocs.yml

@@ -51,6 +51,7 @@ pages:
   - 'Postfix':
       - 'Anonymize Headers': 'u_e-postfix-anonym_headers.md'
       - 'Disable Sender Addresses Verification': 'u_e-postfix-disable_sender_verification.md'
+      - 'Max. message size (attachment size)': 'u_e-postfix-attachment_size.md'
   - 'Dovecot':
       - "Expunge a User's Mails": 'u_e-dovecot-expunge.md'
       - 'More Examples with DOVEADM': 'u_e-dovecot-more.md'