Other sections in this FAQ:
Headers
I would like add some custom headers to selected outgoing mail based on a specific domain and the subject line.
Is it possible to have Exim add a header to only certain local parts of outgoing mail?
How can I remove some part of the Received: header?
How I can insert the PGP header line using Exim filters?
I know I can use a system filter to replace certain headers in messages, but how can I add text to existing headers? I want to add [SPAM] to the subject line of messages that appear to be spam.
How do I reject messages that don't have a Message-ID header? How do I fix MUAs that don't supply a Message-ID?