If you are receiving this error, the most likely cause is having the wrong mail server option selected in the panel. The primary choices are remote and local.
This error is often seen when both the sender and recipient domains exist on the same server, but the recipient server has the wrong mail server selected. You can confirm that this is indeed the problem by confirming that the MX record points to a remote mail server, but the panel has the domain stored in its 'local domains' file.
The server is configured optimally to use as few resources as possible, so it will save itself from querying the MX record of a domain if it can. It does this by sending mail locally for any domain listed in its local domains file. If the domain is listed there, it sends the mail locally even if the MX records would route otherwise externally.
To fix, you just need to edit the appropriate file(s) so that the server knows that is must query the MX record and use the results to route the emails externally.
For cPanel via CLI
Remove the domain from the /etc/localdomains file and add it to the /etc/remotedomains file.
For DirectAdmin via CLI
Remove the domain from the /etc/virtual/domains file.
For cPanel via WHM
WHM ⇒ Edit DNS Zone File Select the domain in the drop-down list Scroll to the bottom of the zone file and local the MailServer Selection section. Select Remote Save * Note: You should be able to use the Automaticallly Detect option if the MX points remotey. Typically this error results from having Local manually selected in leiu of the Automatic Detection option.
For DirectAdmin via the Admin panel
Log in as admin. Show all Users Log in as user Go to User Panel → domain.com → MX Records Uncheck the "Local Mail Server" option.
* Important* Make sure that you do not use the mail subdomain as the MX record for remote domains on DirectAdmin servers. The server routes this MX subdomain locally regardless of whether it points to a remote location. You must remove the mail subdomain as the MX record and use a different MX record. That's it!