[SMTP] requires that the HELO/EHLO argument be a valid name for the
SMTP client host.
Isn't the FQDN for a host the host name "dot" the domain name?
Only if you define "host name" and "domain name" that way. It would be
logically consistent to do so (though somewhat confusing, because "host
name" and "domain name" are already in wide use to mean other things).
Defining them that way is really just giving names to a heuristic. The
"domain name" determined that way will be what you want for a
substantial fraction of your mail clients, but by no means all (well,
unless you have a remarkably unusual mail stream).
The host gets its name after some buddy edits the zone file. Which
zone file? The domain's one.
That amounts to defining a host's "domain name" to be the next zone cut
at-or-above it in the DNS hierarchy. That's a consistent definition,
and useful as a heuristic, but will fail in enough cases it's no better
than that. (Fail meaning give you something other than what you want.)
Actually, it amounts to that for most hosts; for glue nameservers, the
"which zone file" heuristic gives a different domain than the "next
zone cut" one. But neither one is better than a heuristic. (Glue
nameservers usually aren't mailservers for mid- to large-size sites,
but for small private sites, it's not uncommon for everything to be on
the same machine.)
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