Subject Line Manipulation

[Inthereal]

Hello,
I would like to control (or add) to a subject line text.

This is for a server that virt hosts approx 20 domains.  I would like to
be able to add information to all incoming mail for one particular domain.
Is there a qmail trick for this, or do I need to go through a pl script?

Thanks in advance.  So far I have been unable to find anything like this.

Derrik