email problem - help

It began yesterday. I can no longer send or receive any emails from my server. I use squirrelmail and it responds very slowly. Nothing is being received and when I go to send an email I get an error that says:

Requested action aborted: error in processing
Server replied: 451 4.1.8 Domain of sender address admin@deeplist.com does not resolve

I have done everything I can and I still can not figure it out. The server itself is in perfect working condition and still functions fine, but squirrelmail responds very slowly.

Any suggestions? I need help.

 

 

 

 

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