[cobalt-users] Error in maillog

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Hello,

Can anyone tell me what this message in my maillog means?

May 12 16:21:46 www sendmail[3308]: QAA03308: ruleset=check_mail,
arg1=<eerrr@xxxxxxxx>, relay=207.153.203.196], reject=451
<eerrr@xxxxxxxx>... Sender domain must resolve

 I have thousands of them one right after another.  I had to shutdown
sendmail last night when it was overloaded with this same error.  It just
appeared again today.  Once I restart Sendmail it goes away. Any way to stop
this from happening again?

Thanks in advance.

Best Wishes,
Scott