Well, don't bother with Hotmail. Highly unlikely someone using MorphOS might use it. It's becoming more and more heavily tied with Microsoft technology anyways.

I fear a similar issue might be with MobileMe web mail -- again not very likely to be used by someone which is using MorphOS and it's also not free (well, for me it is but...).

What is bothering me now is that my SimpleMail cannot send emails to some other domains as well using IMAP4. For example, I tried emailing SimpleMail developer [mail@sebastianbauer.info] to suggest something and I got the same RCPT failed message. I don't know why is that happening. Could it be a MobileMe issue. But IMAP is an open and very well known protocol... It still sends emails to my MobileMe accounts so I am puzzled.