One user/One sender can’t email

This is driving me mad. One user can’t receive email from one sender.

The user can send to the sender, but when the sender tries to reply, they get a nondeliverable message. Says, “There was a SMTP communication problem with the recipient’s email server. Please contact your system administrator.”

The user can receive email from everyone else AND the sender can email anyone else in our company.

Exchange 2003, MS Outlook 2003, using Securence spam filtering service. The senders email never gets to our Exchange server AND doesn’t show in the Securence quarantine either.

What should I look at? Is there a problem on my end or the senders?