[IMail Forum] unknown user

[Waheed Malek]

Hi,   One of our customers has a virtual domain, he told me that he is receiving the message "unknown user <email.domain.net>" when he sends to a specific email although he has sent and received from this email many times before. This problem does not exist if he sends to this user from yahoo or hotmail. I have tried to send to this email address from my email on another domain that is not virtual but on the same IMail server, and I have got the same result, also when I sent him from yahoo he got received my message. Can you help me please.   Thanks   Waheed Malek