The limits of some anti-spam precautions
In some quarters it is quite popular to do things like refuse email
if the sending machine doesn't have valid reverse DNS or doesn't use
a valid domain name in
HELO). It's also popular to tell
people that everyone should do this, for various reasons.
(Sometimes it's even popular to grumble about how all of the laxness of mailers about this sort of stuff has helped enable the spam epidemic.)
Setting aside all of the other reasons why these things may not be a good
idea, it is worth pointing out that the only reason that these
precautions work now is that not very many MTAs are using them. In much
the same way that spammers once used invalid domains in the envelope
sender address and now almost never do (because large MTAs started
checking that), spammers are perfectly capable of adopting to use valid
EHLO names and to only sending from machines with valid reverse DNS,
if they actually need to. Indeed, the fact that the spammers don't
bother to do any of this is a strong sign that only an insignificant
number of MTAs use such precautions today.
(The history of bad domains in
MAIL FROMs is a great example of this,
in fact. It used to be a great way to get rid of a bunch of spam, until
places like AOL (which was then an important spam target) started doing
it. The next thing you knew, spammers were using real domains. I
wouldn't be surprised if spammers adopted faster than real domains to
the new reality.)
Or in short: spammers are lazy, not stupid (at least in the aggregate).
The corollary is that if you find an anti-spam heuristic like this that works for your email, you should not try to get other people to adopt it. The worst thing you could possibly do for your spam load is to persuade a significant number of MTAs to get more picky in what they accept.
(There is probably already an aphorism somewhere that says 'any widely adopted anti-spam measure will be actively defeated by spammers if at all possible'.)