HELO/EHLO checking:

For example, spam can be greatly reduced by a number of simple checks confirming compliance with standard addressing and MTA operation.

In many situations, simply requiring a valid FQDN in the SMTP EHLO statement is enough to block 25% of incoming spam.

* Refusing connections from hosts that begin transmission prior to presentation of the receiving host's HELO banner
* Refusing connections from hosts that give an invalid HELO - for example, a HELO that is not an FQDN or is an IP address not surrounded by square brackets.................
wikipedia