One of my clients is having issues with spam. It apears one of his PC's is infected with a virus and spamming emails out, however the issue is the bounced back messages. For example, he is spamming emails from info@[B]domainA[/B].co.uk However when looking at the mail queue headers, I can see the following. Received: (qmail 20154 invoked from network); 26 Aug 2012 23:04:06 +0100 Received: from unknown (HELO pyrwpvp) (x.x.x.x) by domainB.co.uk with ESMTPA; 26 Aug 2012 23:04:04 +0100 Any suggestions on why it would be doing this? Domain B is one of my customers, however i don't understand why it is showing in the email returned headers of Domain A?