We have a problem receiving some emails.
When some people try to send to us they get the following failure message:
Subject: Mail delivery failed: returning message to sender
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:
SMTP error from remote mail server after end of data:
host mail-fwd.london21-verio.com [18.104.22.168]:
email@example.com is a valid user with a mailbox on our domain.
The host listed is the entry in our mx record.
we're then picking the emails using pop3.
Can anyone advise me on a) where is this message being generated ? ie who's server and b) what can we do about it?