Exim 500 Firewall Error

[Chocobo22687]

We currently have 2 servers, one being the primary and one purely being a backup DNS and Mail server (running DNSonly and Exim).

Everything works as expected when the primary server is unavailable the mail is sent to the backup mail server, and forwarded on once the primary is back online.

The only issue we have is that the error message below is randomly generated from some emails.

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:

[email protected]
SMTP error from remote mailer after MAIL FROM:<[email protected]>
SIZE=26971:
host recieversdomain.com.au [xxx.xx.xx.xxx]: 500 Firewall Error

When the primary is back online and the secondaries mail queue processed 1 in 10 messages (roughly) is getting dropped at the secondary and NDR'd with that error, the rest are being sent through without any problems and being received and delivered at the primary.

There are no blocks in the receiving servers software firewall (Configserv's CSF) and the IP's are white listed.

Anyone have any further ideas? Looking around online hasn't really drawn anything conclusive.

Thanks in advance!