[Mailman-Users] tweaking mails

On some of the mailman lists I manage, there might be some value if when the mail comes in to mailman, if I could parse the mail looking for key phrases or terms and based on what is found, either tweak the mail contents and/or insert some additional information above or below the mail's contents before mailman sends this updated mail message out to the subscribers.

The information added to the mail could be based on the mail's contents and/or information maintained in a file that would be tracking the info that was added in previous mails.

Is this possible? Issues to be aware of in doing this?

Thanks