[CentOS] sendmail Deferred mail - reason unknown

> #1 - turn your sendmail logging/debugging setting up as high as
> it will go for just long enough to capture some of these events.
> (then turn it back to its previous setting)
>
> #2 - try using script and then telnet to capture an SMTP session
> (Done by hand) with the MTA at the receiving end.  This can be a
> little tricky and requires a good understanding of how SMTP
> transactions are done so you can emulate it by hand.

Thank you all for the suggestions, I think we may have pinned down the
problem.  This list is a great resource, props to all the experienced
users who put in time answering questions here.  :)