We have been having problems sending emails from our mail server (running Kerio) to a particular domain over the past few weeks.  We kept receiving bounce emails with the following:

550 Rule imposed. Sender using invalid hostname 
in greeting as per RFC specification.

After trying all manner of solutions, it turned out to be a problem with our Kerio setup.  Lots of mail servers are being configured to only accept connections from Fully Qualified Domain Names (FQDN) – we had our mail server setup without a fully qualified address.  So we needed to make the following change:

example.com => mail.example.com

To apply this change to our Kerio mail server, we simply changed the setting in Kerio using the following steps:

  1. Log in to the administration console
  2. Click Domains
  3. Change the Domain to a fully qualified domain (e.g. add “mail” to the start”)
  4. Click Apply
  5. Problem solved

If you are using a different mail server, consult the associated documentation and ensure you have the mail server setup with a FQDN.