Sorry to ressurrect this old thread, but I'm having what seems to be the same problem. I can receive mail on both of my .Mac accounts, but I can't send. It says (in Japanese, since I'm running the stock rom) "Host not found. Please check your mail server settings." Changing the SMTP port number to 587 as suggested in the Apple Knowledgebase article above didn't do any good, but since all of my other computers work fine (Windows and Mac) I can't believe it is an ISP issue. BarryW, did you ever solve this problem? If not, are there any other IMAP-capable email clients that might work?
Edit: Well, I guess I spoke too soon. It turns out that for some reason my wireless connection on the Zaurus had stalled just when I decided to try sending mail for the first time, which was leading to the error. It looked like it was connected, but it apparently wasn't. Reconnecting did the trick.