How to Configure a Postfix Disclaimer

Many organisations are required to add a legal disclaimer to their email - confidentiality notices, regulatory statements, or GDPR-related text. The catch is that Postfix has no built-in disclaimer feature. As a mail transfer agent it routes mail; it never edits the body of a message.

To configure a disclaimer in Postfix you add a component to the mail path that rewrites messages. Below is the traditional alterMIME method and its limits, then the simpler, rule-based way with MSH Postfix Milter.

The manual way: Postfix and alterMIME

The classic approach pipes mail through a content filter that callsalterMIME. You define the filter in master.cf:

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}

The script then runs alterMIME against a disclaimer file, and you typically keep a list of which sender addresses should get the disclaimer:

# only disclaim mail from addresses listed here
if grep -qi "^${from_address}\$" /etc/postfix/disclaimer_addresses; then
  /usr/bin/altermime --input=in.$$ \
    --disclaimer=/etc/postfix/disclaimer.txt
fi

It works for simple cases, but the limitations are well known:

• Plain text only, in practice. Reliable HTML disclaimers across all MIME shapes are hard with alterMIME.
• Coarse targeting. Limiting it to outgoing-only or to external recipients means maintaining address lists and extra logic by hand.
• Duplicates on replies. The disclaimer stacks up through a thread unless you build detection yourself.
• Two submission paths. If users submit on port 587/465 you must wire the filter into those services too, or some mail escapes without the disclaimer.
• File-and-reload workflow. Every wording change is a server edit.

The easy way with MSH Postfix Milter

MSH Postfix Milter adds disclaimers as a milter, configured from admin panel. You write the disclaimer once, decide which messages it applies to, and the milter handles HTML, plain-text alternatives, positioning, and duplicate detection for you.

Configure it in four steps

1. Install the milter and connect it to Postfix with smtpd_milters - this covers all submission paths at once.
2. Write the disclaimer as text or HTML in the Administrator Panel.
3. Scope the rule - for example outbound mail only - and choose where the disclaimer sits in the message.
4. Test and enable with the built-in rules tester.