FAQ / Server Products

What does it mean when my mailserver reports "501 5.5.4 Invalid domain"?
In Exchange this can refer to the HELO line you have configured to use when talking to your mailserver.

Be sure that the HELO line configured is just
HELO domain.com
or
EHLO domain.com
or the default
HELO <domain>
(which replaces <domain> with your domain name automatically)

Do not use any special characters in the HELO line or it will be rejected. For example this is INCORRECT:
HELO <mydomain.com>

There are also the following guides: