[CentOS] sendmail Deferred mail - reason unknown

[jkinz at kinz.org]

On Mon, Dec 15, 2008 at 10:15:15AM -0600, Sean Carolan wrote:
> exmx1 : Dec 14 05:28:39 exmx1 sendmail[31574]: mBDIYhEa031574: to=<
> customer at example.com>, delay=00:00:00, xde
> lay=00:00:00, mailer=esmtp, pri=41874, relay=mail.example.com., dsn=4.0.0,
> stat=Deferred
> 
> Unfortunately there is no reason given for the "Deferred" status such as
> "Server unavailable", etc.
> 
> Do you have any ideas how I can begin to troubleshoot this?

Hi Sean,
Two suggestions, 
#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.

I would focus on hat the receiving end does when you send it a
length that is > 1 MB.

 Jeff Kinz