Hey uxbod, all mail is html by default (the screams outweigh the benefits of plain text), and users can definately not be counted on to switch when faxing. It seems like it may just be the signature thats causing problems actually, will post back on what ive done.
So far I have python wrapper that gets the text part of the multipart/alternative part of the message, as well as any attachments and passes those to sendfax (which will die if its a doc etc). Seems one way to do doc etc conversion is to run a headless openoffice as a server, and feed stuff to this for conversion - really dont like the idea as ill have to add another few gigs of ram to the server to keep ooo happy
Its looking more like a winprint-hylafax solution is the way to go as everything ends up at the server as ps, and keep the mail-fax gateway for users with clue >0