> Note that in this day of MIME and multipart messages, attachments, digitally
> signed email, encrypted email, and everything else, it is generally a very bad
> idea to try to append an arbitrary block of text to any message -- even an
> innocuous footer. For this reason, we ask that you consider _very_carefully_
> whether such an action is needed before mangling the mail of your users.
Having said all that, it's not that hard to detect a MIME message and
append another component. If it's not MIME then a textual appendage
usually does the trick. Unmarked HTML email can be a bit tricky, but
even this is tractable. So in all there is just three relatively
straightforward cases that cover the vast majority of email.
Furthermore, most encryption and signage software uses textual markers
to tell it where the signed/encrypted section ends so an appendage
after that is unlikely to do damage. Even then, if any damage is done
it only tends to reduces the security risk by not decrypting, or not
recognizing the signature.
Finally, all of this is usually done at a single point - the outbound
MTA - so it's not too difficult to catch unhandled cases and deal with
them on a per-user basis.
> Note that there can (infrequently) be good technical reasons for doing so.
> Adding several kilobytes of incomprehensible legalese is _never_ one of these
> reasons.
Well, I think it's always presumptious to define what is appropriate
email content when sent from one consenting MTA to another.
Regards.
