We have one customer who is constantly getting rejections when sending to one particular domain. The error message is "Remote SMTP Server Returned: 550 Your HELO/EHLO must be contain FQDN or literal address(RFC-2821)". The domain is Fully Qualified, DNS setup for email is mail.domainname.com.au. The only difference is the website itself is on a different server than email so it's A record points to a different IP address. Not sure if this is the issue though. The email hosting is on a Plesk server which also handles DNS. Any tips on where I should start looking to see why they are getting rejection notices? MXToolbox doesn't find any errors either.