When I ran a FBB setup I did see this work and I have read Ricks comments about having to save the unsent part of the message on the CMS and the problems with that but that is not how FBB did it.  The message was save as a binary file on the client in the mail.bin and then uncompressed in to the mail folder.  If the transfer was broken the portion of the received message was saved by the client.   Upon reconnection after the exchange of the MID's
the client would sent a Y xxxx    The xxxx would be the size of the binary file it saved from the first try for the message with that matching MID.
The CMS would have to be able  to  skip down to the xxxx byte and start sending at that point and the client has to append the new data onto the file it has in the mail.bin.  Seeing that the CMS is sending all binary files from the outside looking in this looks like a great way to save a lot of air time.   73 Steve N9LOH