I have a domain with about 20 aliases. These all forward to the individuals personal mail accounts. There is one account that when I email them, I get the mail bounced back as below;
This is the mail system at host box.unitymail.co.uk.
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
<firstname.lastname@example.org>: host ali.uk.net[22.214.171.124] said: 550-Please turn on
SMTP Authentication in your mail client, or login to the 550-IMAP/POP3
server before sending your message. box.unitymail.co.uk
550-[126.96.36.199]:49531 is not permitted to relay through this server
without 550 authentication. (in reply to RCPT TO command)
I don’t understand why their email server thinks I am trying to relay through their machine. Anyone have any observations on this?