Dumb though it may seem, a hosting company I just moved to (IXwebhosting) uses POP-before-SMTP instead of Authenticated SMTP, which they simply don't support. You can see where I'm going with this. Odd though it may seem to use this sort of defence when the source web server and the SMTP server have the same domain name, but there it is.
You get a standard SMTP ish error message, of which the 5.7.1 bit is the "I'm not going to relay for you even though you're me" message (see below).
So the request is for a little mod... you could use the same UI to enter POP details as you do today for authenticated SMTP, but add a little check box which says "don't use authenticated SMTP, POP, then wait a second, then SMTP". That would do the job for IXwebhosting.
In the mean time, I turned off all the email features so I don't hit this error, which fills up the error log rather rapidly. I coded around it on everything else, but I don't have the tools to take the lid off YAF, hence this post.
Still, it's good that the software copes rather nicely with this - thanks.
Error sending emails to users
System.Net.Mail.SmtpFailedRecipientException: Mailbox name not allowed. The server response was: sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1) at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at yaf.pages.ForumPage.SendMailThread()