Using OE6 I was sending an e-mail to an address I've often sent to before,
using my AT&T Internet connection, and instead of the e-mail being sent I
got this message (I've changed the e-mail address to a generic name with the
same form and the subject to "Subject name"):

The message could not be sent because one of the recipients was rejected by
the server. The rejected e-mail address was '. Subject
'[Subject name]', Account: 'ipostoffice.worldnet.att.net', Server:
'imailhost.worldnet.att.net', Protocol: SMTP, Server Response: '450
[TEMPFAIL] destination not valid within DNS', Port: 465, Secure(SSL): Yes,
Server Error: 450, Error Number: 0x800CCC79

This happens from time to time: addresses I've successfully sent to many
times will suddenly cause this error message, and then at a later point I
can send to them again. It's not a permanent issue, but it's very annoying
when it happens.

What is the explanation for this?