A friend of mine just told me that he can't send mails to accounts on one of my servers via the SMTP server provided by his ISP.
The error message in the bounce he gets reads:
Hi. This is the qmail-send program at aon.at. I'm afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out. <email@example.com>: CNAME lookup failed temporarily. (#4.4.3) I'm not going to try again; this message has been in the queue too long.
Any ideas what could be the reason for this?
I have double-checked the DNS records for my domain, but they seem perfectly fine, and from any other mail servers I tested, delivery works flawlessly…