My question to all linux wizards. I am though very new to linux, I was able to setup ISPCONFIG 3 on my ubuntu 10.04 64bit desktop. Every thing in the server is running fine. All services are running fine as well. The problem I am facing is I am able to send mails from outside to my mail addresses configured in ISPCONFIG, but I am not able to send the mails to outside users. I am getting the error like Code: From: Mail Delivery System <[email protected]> To: [email protected] Subject: Undelivered Mail Returned to Sender Date: Wednesday 28 July 2010 02:54:00 IST This is the mail system at host server1.mywebhostings.biz. 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]>: host gmail-smtp-in.l.google.com[220.127.116.11] said: 550-5.7.1 [18.104.22.168] The IP you're using to send mail is not authorized to 550-5.7.1 send email directly to our servers. Please use the SMTP relay at your 550-5.7.1 service provider instead. Learn more at 550 5.7.1 http://mail.google.com/support/bin/answer.py?answer=10336 b21si2383474ana.176 (in reply to end of DATA command) I did check the link given http://mail.google.com/support/bin/answer.py?answer=10336 In this URL link it mentions that sending IP address does not match the sending domain. I couldn't locate the problem except that the sender host is mentioned as server1.mywebhostings.biz and not orissaitbazar.com. could this be the problem. Both the domains that host server1.mywebhsotings.biz and orissaitbazar are on same ip address. Please note that I am using static ip and not dynamic ip. I have looked everywhere to find the solution, but I couldnot find a solution.