Postfix Disclaimer for Outgoing Mail

Adding a disclaimer to every message is easy to get wrong: nobody wants a lengthy external confidentiality notice on a one-line internal message between colleagues. What most organisations actually want is a disclaimer on outgoing mail only. Postfix does not add disclaimers at all on its own, and a naive content filter runs on all mail - inbound and internal included.

The key is telling outbound mail apart from the rest. This page shows why the simple filter approach over-applies the disclaimer, and how MSH Postfix Milter targets outgoing mail precisely with a message-direction rule.

Why the manual filter over-applies

A typical alterMIME content filter is attached to the smtp service inmaster.cf, so it processes everything that passes through:

smtp      inet  n       -       y       -       -       smtpd
  -o content_filter=dfilt:

dfilt     unix  -       n       n       -       -       pipe
  flags=Rq user=filter argv=/etc/postfix/disclaimer -f ${sender} -- ${recipient}

To restrict it to outbound mail, admins resort to maintaining an allow-list of local sender addresses and testing each message against it:

# every local sender that should get the external disclaimer
alice@example.com
bob@example.com
sales@example.com

The trouble with that:

• It is a maintenance treadmill. Every new or departing employee means editing the list.
• Direction is only approximated. Matching on sender address is not the same as truly knowing a message is leaving the organisation.
• Internal mail can still match. A listed sender emailing a colleague gets the external disclaimer anyway.
• Submission ports need separate wiring. Authenticated submission on 587/465 must be filtered too, or outbound mail escapes undisclaimed.

Outbound-only disclaimers with MSH Postfix Milter

MSH Postfix Milter understands message direction directly. You add a message-direction condition to a disclaimer rule and it matches outgoing mail only - no address lists, no per-port wiring, and internal mail stays clean.

Configure it in four steps

1. Install the milter and connect it with smtpd_milters - one hook covers all submission paths.
2. Write the disclaimer as text or HTML in the Administrator Panel.
3. Add a message-direction condition so the rule matches outgoing mail only.
4. Test and enable - use the rules tester to confirm internal mail is left untouched.