Yes, at this point I am also seriously suspecting something with MobileMe POP / IMAP implementation since I only use Apple clients (OS X Mail, iPhones mail, iPod touch mail) and they all use "automagical" MobileMe setup with and probably bypass this more conventional IMAP/POP setup.

However, if that is the case, more clients would have issues and it would be a huge issue for Apple, don't you think So I am still puzzled and will play more with this (in the meantime i tried SimpleMail last night with POP and still have the same "RCPT failed" issue).

Maybe my best bet is to wait for MobileMe over Web to start working in OWB. I already offered Fab to send request/response logs if possible and if he is interested in debugging it (I know I am an edge case here again and there is probably no any incentive to make MobileMe mail work in OWB any time soon (if, indeed, ever), but sometimes people are just curious why stuff does not work -- if that's the case, I am open for collaboration).