Help! I can't send email!
One thing which might be causing this trouble - we have spam protection software installed which detects your IP address when you check your email, and stores it in a cache. Then when someone goes to send email, it checks to see if that IP address has been validated.
This is for your security - only someone with your email password can send email through your smtp. (So a spammer can't masquerade as you.)
So, if you check your email, wait a little over one minute (it refreshes every 60 seconds), then try to send, you should be ok.
If this is still not working, submit a trouble ticket.