I have a client who has been complaining that they can not receive email from a specific domain. I have run multiple successful tests from other servers from outside the client's network and email goes through fine.

The tech from the sending domain that can not seem to get mail to my client supplied me with a snippet from his maillog. (SEE BELOW)

** The question is...why is the 'to=' entry missing? This is something that the tech should be answering for me but the client nor this domain knows what is going on.

Shouldn't that entry say the destination..IE, 'to=user@domain.com' ?

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I stripped out the domain name and IP below.
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Feb 15 08:26:00 monitor1 sendmail[24550]: m1EJPwtk024548: to=, delay=00:00:02, xdelay=00:00:02,
mailer=esmtp, pri=300972, relay=mail12.domainname.com.
[70.xx.xxx.xx], dsn=5.1.3, stat=User unknown

Feb 15 08:26:00 monitor1 sendmail[24550]: m1EJPwtk024548: to=, delay=00:00:02, xdelay=00:00:02,
mailer=esmtp, pri=300972, relay=mail12.domainname.com.
[70.xx.xxx.xx], dsn=5.1.3, stat=User unknown