I have the same mailer-daemon:
This is the Postfix program at host oldcourse.ridderikhoff.net.
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 Postfix program
<firstname.lastname@example.org>: Host or domain name not found. Name service error for
name=apple.com type=MX: Host not found, try again
Very strange, as ittakes approx a week to get this messageand I do not see a patern, meaning that some seem to go through and some get bounced.
Any ideas how this could be resolved?