I am going crazy about this error message.

for some of the domains, I am receiving this error message;

> 
> Reason: Remote SMTP Server Returned: 550 5.7.1 Unable to relay for
>  info@example.com
> 
> 
> 

I am using MailEnable on windows server 2008 r2. I have just migrated the whole domain from windows server 2003 and the e-mails worked just fine there.

What would solve this problem?