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=<firstname.lastname@example.org> temporarily rejected RCPT <email@example.com>: remote host address is the local host
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.
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: