Spam filter services - 421 on supposed bad HELO/EHLO

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220 XXXXXXX ESMTP DumbWave dork4005
HELO simplehostname
421 Offline: HELO/TLD
Connection closed by foreign host.

Please read:

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.