Hiya, I know this doesn't exactly fit your problem, but I just wanted to voice that I've had a similar issue (likely a different problem) and throw this story out there in case it helps anyone else.

When I first got the n900 I tried to set it up using exchange from gmail. This resulted in email coming in, but nothing going out. I thought, "OK, no problem, I'll just add gmail as an IMAP account and use that for sending". This did not work, still the same problem trying to send email, even though I was trying to send from the IMAP account.

I took it one step further and disabled email sync on the exchange account, so then I was just syncing calendar events from gmail and using an IMAP connection for sending and receiving email. STILL wouldn't send! I tried using both IMAP to gmail and Nokia Messaging with my gmail account, and no emails would send until I deleted the exchange account.

How I finally got it working:

I used nueva sync to check my gmail calendar, but set it up on nueva sync that my account used an email address different from the one I intended to send email from. (Nueva sync checks XXXX@gmail.com for calendar events, but on the N900 the exchange account name is YYYY@gmail.com)

I combine this with a Nokia Messaging connectiont to my gmail account. (XXXX@gmail.com)

In this way I've got MOSTLY-reliable push email, and google calendar sync.

It was a frustrating problem, and I'm still not sure what the exact cause was.

J