Seems to be a simple problem, but I can not figure this one out: I have a Ubuntu server setup using the perfect setup for 6.06 and with Ispconfig. This server's hostname is server.anaselectronics.com. anaselectronics.com is on the main IP of the system. On a 2nd IP, I have asuslaptops.net. Both of these sites are setup through Ispconfig. I can send and receive emails to both domains successfully from: - from email address of one domain to the other domain - from php scripts on the server using the php mail() function - from my gmail account The problem I have occurs when sending email from a different server housed at a different datacenter. The hostname is server.nyxmotorsports.com and the domain being used for sending the email is case-mod.com. When sending email from email@example.com to firstname.lastname@example.org, the emails go through just fine. When sending email from email@example.com to firstname.lastname@example.org, I get the following errors: If sending from thunderbird, it says If sending mail from telnet mail.case-mod.com 25, it accepts RCPT TO: <email@example.com>, but RCPT TO:<firstname.lastname@example.org> returns If sending from the php mail() function using a php script on case-mod.com server, emails sent to email@example.com go through. Any emails sent to firstname.lastname@example.org do not come through and bounce back after few hours: So basically, it seems the site setup on the main IP of the server can receive emails without any problems. The problem I am having here seems to occur on a domain that is setup on a secondary IP in Ispconfig. Thanks in advance for any help.