I thought one indicated "we receive no mail" by establishing a valid
MX record that points to a machine with the smtp port open and a
silly little daemon that gives 552 errors in reply to all MAIL FROM
commands listening on it. (would that be a viable commercial
service?)
Depends. I know of some domains that get several thousand connection
attempts per second, all spam, when they have an MX. How much
bandwidth does your daemon have and how many simultaneous connections
will it handle?
Several thousand connections per second, at (say) 100 bytes of traffic
each (banner, HELO/EHLO, response, MAIL, response, QUIT, response - if
the responses are short, I don't think 100 bytes total is far off)?
Several hundred thousand bytes per second. Not over a 10Mbit pipe's
capacity, though approaching it (perhaps even a little over it, given
the various layers' overhead). Lots of hosting centres can do a 10Mbit
pipe no problem.
Simultaneous connections? After doing the mail shim for work, I
believe I could build an SMTP daemon which does that; based on some
back-of-the-envelope estimates, simultaneous connections might be a
problem, but probably not requiring more than a little kernel tuning
(and maybe tarting up the machine with RAM, though given modern ideas
of reasonable RAM load, possibly not).
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