Exim shows error "remote host address is the local host" when processing mail

Symptoms

When email is sent to an address on the server, it is rejected with an error similar to the following:

H=host.server.tld [192.168.1.1]:19338 X=TLS1.2:ECDHE-RSA-AES128-GCM-SHA256:128 CV=no F=<user@domain.tld> temporarily rejected RCPT <user@localdomain.tld>: remote host address is the local host

Description 

This error is caused by the domain being listed in the /etc/manualmx file on the server. When accounts are moved with the Live Transfer function to a new server, the old server will attempt to forward messages to the new system. In certain cases where the account was moved back to the initial server, the forward can still be in place leading to this error.

Workaround

To disable the email forward remove it from the manualmx file via whmapi1:

whmapi1 unset_manual_mx_redirects domain=example.com

More information on managing the manualmx file can be found here:

How do I add or remove a domain from /etc/manualmx?