DevHeads.net

A problem I'm not sure how best to solve

I have a perplexing puzzle thrust upon me.

Consider the following:

Oct 8 15:55:33 minbar postfix/smtpd[7422]: NOQUEUE: reject: RCPT from
rs230.mailgun.us[209.61.151.230]: 551 5.1.8 < ... at mg dot pluspora.com>:
Sender address rejected: Domain not found;
from=<bounce+db1162.5fcd4c-alrekr=caerllewys. ... at mg dot pluspora.com>
to=< ... at caerllewys dot net> proto=ESMTP helo=<rs230.mailgun.us>

mailgun.us is connecting with a good HELO, and appears to be authorized
to send mail on behalf of pluspora.com, but the mail has a sender
address that is bad because mg.pluspora.com does not resolve in DNS, and
so the mail is rejected.

I want to TEMPORARILY (I hope) whitelist <a href="mailto: ... at mg dot pluspora.com"> ... at mg dot pluspora.com</a> as a
sender address as long as the mail is being sent by mailgun.us.

How would you do it?