We host our own e-mail and use Exchange Server 2003 in addition to TZO's Outbound Mail Relay Service. There are a small number of domains we cannot email to that get bounced with the following error:
SMTP error from remote mail server after RCPT TO:<firstname.lastname@example.org>:
host etrnmx.superonline.com [184.108.40.206]: 550 5.7.1 Unable to relay for email@example.com
I can't tell if this is an issue with my MX records for our internal domain or if it is an issue with TZO's setup. It happens to us probably 1% of the time but it's still irritating and I'd like to fully resolve it. Thanks for any input or direction.