I posted a similar problem, I was still typing mine when you posted yours :-)
Anyway, I managed to get my email to send by retrying it. Many times. Over a period of half an hour. I've sent a couple more since then, same problem, just keep retrying and it'll work eventually.
It's also worth noting that every time you try it and it fails, it puts the ntlhome signature on the bottom of the mail, so you need to remove it whenever you retry or the email you send wil have a couple of dozen signatures
