Best method to handle invalid HELO hostnames?

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Best method to handle invalid HELO hostnames?
\ Justin Piszcz (16 Mar 2008)
. \ mouss (16 Mar 2008)
. . \ Justin Piszcz (16 Mar 2008)

3 msg	How to block certain tld's

Subject:	Best method to handle invalid HELO hostnames?
Group:	Postfix-users
From:	Justin Piszcz
Date:	16 Mar 2008

Some examples:

<client=211.55.48.96> <helo=samsung-yg23rdm.kornet>
<client=121.170.104.24> helo=<userodd92mbtcb.domain.invalid>

I realize there is the reject_helo but we all know there are misconfigured
mail servers out there and that could reject legitimate e-mail.

One could go through the list of TLDs and add them into a list and then if
the TLD did not match anything on the list or a regexp then reject the
message.

/\.com$
/\.net$
/\.info$

But new TLDs get added every now and then and that would need to be kept
up to date.

Perhaps a regex would be better?

/^\.[a-zA-Z]{2,4}/

$ echo test-domain.gr | grep -E "\.[a-zA-Z]{2,4}"
test-domain.gr

Or per this:
http://en.wikipedia.org/wiki/Top-level_domain

One could add these to the HELO checks to reject:

RFC 2606 reserves the following four top-level domain names for various
purposes, with the intention that these should never become actual TLDs in
the global DNS:

* .example - reserved for use in examples
* .invalid - reserved for use in obviously invalid domain names
* .localhost - reserved to avoid conflict with the traditional use of
localhost
* .test - reserved for use in tests

Example:
No blocks there.

Invalid:
donna645b6fcff.domain.invalid
lastxp.domain.invalid
user023a8b376a.domainnotset.invalid
user023a8b376a.domainnotset.invalid
userodd92mbtcb.domain.invalid
userodd92mbtcb.domain.invalid

Localhost:
localhost.scwe4.on.cogeco.ca
localhost.scwe4.on.cogeco.ca
mail.localhost
mail.localhost

Test:
test
test

Was curious how others dealt with this?

Thanks,

Justin.