I just spent the afternoon at a client's hacking qmail+courier to work storing mail on a NT server.
Why? Well, migration issues, from NT to Linux.
And what had to be hacked? Lots.
You can't chmod over SMB
You can't link() over SMB (you can unlink(), though), so in order to have atomic message moves from Maildir/tmp to Maildir/new and whatever, rename()
Good thing this was a single-UID qmail system: you can't chown either ;-)
Courier's metadata is created on Maildir/tmp and moved: same problem as above
Cranky permission checks fixed via uid,gid,fmask mount options.
It had to work with a canned webmail thing, which expects all mail to be in one folder on disk, so Maildir/cur and Maildir/new have to be symlinks to a single folder.
Since you can't have symlinks on SMB, Maildir is local, but subdirs of it are symlinks to the SMB-mounted thing
Then courier's pop doesn't work because you can't rename from Maildir/tmp (over SMB) to maildir/courierpopwhatever (local), so hack courier to use mkstemp or whatever
Then courier got each mail twice. Looks like it sees two UIDLs for the same file, depending on whether it's read or not (my guess: because we are mixing Maildir/cur and Maildir/new on one folder). Make courier use alternative UIDL format and it works.
And so on. About two hours, and I am pretty sure noone has ever done something like this before. On the other hand, when noone has done something like this before, they usually had a very good reason!