A very handy format where a BBS would store all of the new messages in the forums that you wanted to read into a ZIP (or other compression type) file, send it over to your side, so that you could read it with your Offline Mail reader. (SLMR and OLX are two examples: OLX being the commercial and more mature child of SLMR's codebase). It would then accept your upload of any messages that you wrote offline, and put them in all the right places.
Many BBSs supported them directly (like Telegard, Maximus BBS, and Renegade BBS) and others supported them through a door program like Blue Wave (like TAG BBS).
It was very handy that you could use your handy dandy text editor to work on your replies to messages instead of using whatever hack of an editor that the BBS software might provide you for working online. And all BBSs started to look the same when you read everything through the same offline mail reader ... but I kind of thought that was good. The message became the whole of the experience, instead of the BBS's presentation of the message. It was nice that suddenly, it didn't matter that you were on a WildCat! BBS and you didn't like how it handled X -- you weren't logged on for more than the time it took you to transfer your replies up and get your QWK packet down.
There was a similar format eventually realeased for reading Usenet messages called SOUP. I never really understood the need, since a good newsgroup reader took care of it. I guess it was the first step for us PC people to handle the Usenet, though.