SMTPDiag is a handy tool for tracking down SMTP based issues.  I recently used in a newsgroup posting that was helpful along with a few other times at http://www.orcsweb.com.   It is definitely one of those tools worth having if you have to troubleshoot email issues. 

• Nice short article how to use SMTPDiag.
• Download SMTPDiag.   It doesn't require an install so it can be copied to any machine.
  

Bernard Cheah posting is helpful also.

• http://msmvps.com/blogs/bernard/archive/2004/09/28/14480.aspx