The server response was: 553 sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1)
I'm seeing this problem an awful lot on more than one of my domains. It's not consistent (some days I get the error, other days everything runs fine). Today it started happening on a site that it's worked fine on for the last week.
I have configured my web apps to use mail.mydomain.com, mail.m****here.biz, and smtp.mydomain.com, and have seen this problem under each of those configurations. It's happening in both php and ASP.Net applications. What is the cause of this error?