Hi All,When users send out mail via my server to a very special domain (let say domain.at), they get back the following error. But sending mails to all other domains seems to work.What is the problem for issue?My mail server is exchange 2003 SP2 #5.5.0 smtp;550-Callback setup failed while verifying <example@xx.com> Thanks,Halil

Hi Halil, Based on my research result, the issue mostly occurs that the remote domain blocked your email server due to some reasons. You can identify the issue by check the SMTP protocol log. If I am right (the message is rejected by remote domain), I suggest you contact the remote domain admin regarding the issue. Mike

Hi Mike, Thank you for your suggestions, Iwill contact to remote domain admin. Thanks, Halil