first of all, I'm just a php developer but I have to manage servers as part of the job .. so please bear with my limited knowledge of unix commands/config I have two dedicated servers to manage both have been running fine for years couple of days ago one of them started to show a weird problem with email lets call them goodserver ,illserver goodserver has many domains lets pick gsdomain1,gsdomain2 for example illserver has two domains isdomain1 , isdomain2 illserver has the following weird behaviour I send an email from [email protected] to [email protected] the message doesn't make it and I get the following reply Code: This is the mail system at host localhost.gsdomain1 I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to postmaster. If you do so, please include this problem report. You can delete your own text from the attached returned message. The mail system <[email protected]>: user unknown. Command output: Invalid user specified. Reporting-MTA: dns; localhost.gsdomain1 X-Postfix-Queue-ID: A1F653B0AE X-Postfix-Sender: rfc822; [email protected] Arrival-Date: Tue, 18 May 2010 15:10:07 +0100 (BST) Final-Recipient: rfc822; [email protected] Original-Recipient: rfc822;[email protected] Action: failed Status: 5.1.1 Diagnostic-Code: x-unix; Invalid user specified. however, goodserver sends/receives email fine to/from any other server illserver sends/receives email fine to/from any other server isdomain1 sends email to gsdomain1 fine isdomain2 sends/receives email to/from gsdomain1 fine so the problem is only when I send email from goodserver(gsdomain1/gsdomain2) to isdomain1 what can I do about that?