This needs VPOP3 version 5 or later.

This script allows you to add fixed text to the subject an incoming message, eg so that users can see that a message is an incoming rather than local message (be aware of weaknesses in this strategy - eg forwarded messages, spoofing of the marker, breaking DKIM message signatures, etc)

Add the following to the spamrules_userchecks.txt file. This script file only gets processed on incoming messages (the spamrules_localmail.txt script ends the processing of locally sent messages)

IfMatch subject /\[external\]/
# modification already exists
Else
 AddHeader "Subject: [external] ${Subject}"
EndIf