I was setting up a new server a few months ago, and messed up the DNS records.
This is what happened when a hostname was not specified.
Code:
Hi. This is the qmail-send program at yahoo.com.
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.
<test@domain.com>:
10.20.30.40 does not like recipient.
Remote host said: 451 4.3.5 Server configuration error
Giving up on 10.20.30.40.
I'm not going to try again; this message has been in the queue too long.
At least I think that's what caused it. (Should have posted this when the issue happened and was fixed, not months later. Oops.)
