Sorry, there's a piece of this that I neglected to address in my earlier
On 9/18/20 6:36 PM, Sam Varshavchik wrote:
I didn't intend to dismiss or ignore this input. I believe you when
you say it's been a good spam filter for you.
Courier has an optional setting that can be enabled, that verifies
that "the domain name argument in the EHLO command actually correspond
to the IP address". I have it enabled. It's one of my most successful
spam filters. It's very valueable to me. It's possible that there were
one or two instances in the last 25 or so years when I found out that
this rejected something that wasn't junk, but I don't immediately
recall a single one. I'll stipulate that there might've been one or
two times, and that's a pretty good record.
At some point in the past, this was _not_ a reliable spam filter. To
me the fact that it works as a spam filter today seems like mere
circumstance or accident; I don't see any inherent reason that it will
be a reliable spam filter going into the future. Spammers do learn, if
slowly, so if they have to learn to make sure their EHLO arguments match
their source IP addresses, they'll do that. In the long term, I don't
think this check helps anything.
SMTP has been around for nearly 40 years now (close enough to round
up). Are we intending this standard to be applicable for decades past,
only today as a snapshot in time, or decades into the future? I would
argue that it's the latter, and that SMTPbis should make recommendations
that there's reason to believe will hold up over time.
Separate from all of this, there is an emerging set of (as far as I
know) largely-unwritten "rules" for how to make your outgoing mail
appear legitimate enough so as to not get flagged by spam filters as
often. This is a mess because most of those "rules" are basically ad
hoc and not based on any long-term reliable indicators of message
legitimacy. But these rules seem certain to keep changing, so IMO it
makes sense to keep them out of the (hopefully long-term stable) SMTP spec.
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