220 XXXXXXX ESMTP DumbWave dork4005 HELO simplehostname 421 Offline: HELO/TLD Connection closed by foreign host.
If you do this, it seems you confuse spam filtering / blocking
with email blocking.
Please change the name of your service to email blocking, so that it matches the service
Summary in case this link breaks in future:
rfc2821 - ... An SMTP server MAY verify that the domain name parameter in the EHLO command actually corresponds to the IP address of the client. However, the server MUST NOT refuse to accept a message for this reason if the verification fails: the information about verification failure is for logging and tracing only.