.gwia.domain added to email address

When you do a reply to an external internet message .GWIA.DOMAIN is added
to the email address, is there a setting that causing this to happen ? It
also stores the incorrect address in the Frequent Contacts. If I receive a
message from user@aol.com and do a reply it adds "user@aol.com".GWIA.DOMAIN
to the outgoing email address. Any help resolving this would be greatly
appreciate.

Thanks!!!

Joe