Typically it is only the reverse DNS (PTR) record that is matched.

lmnop: I thought mail servers were set up to refuse HELO from hostnames that don't resolve, to reduce spam?

Any decent anti-spam system will use this as one of hundreds of heuristic decision making points.  Rejecting mail on this one thing along would be extremely boneheaded.

I have no doubt that some mail admins reject on it alone, but they shouldn't.

muppet:

 

I have no doubt that some mail admins reject on it alone, but they shouldn't.

 

Rejecting on this alone used to be quite common practice in the early '00s. These days, more weighting should be given to SPF and DKIM and such bonehead mechanisms not used.

It is explicitly permitted to use address literals in HELO/EHLO and further, "SMTP systems are expected to make every reasonable effort to accept mail directed to Postmaster from any other system on the Internet". Blocking at the very first step violates that requirement IMO.

yitz:

Typically it is only the reverse DNS (PTR) record that is matched.