one verdict for one mailbox: does its MX accept it right now
probe-email answers one question about one mailbox: does the MX of
its domain accept mail for it right now. A single SMTP dialogue --
EHLO, MAIL FROM, RCPT TO, QUIT -- read in lockstep, one reply read
before the next command goes out. Read-only: the dialogue stops at
QUIT, before DATA, so no mail is ever sent.

Silent and rc 0 when the address is accepted. Otherwise rc 1 and a
usage line, rc 2 when the domain has no MX, rc 3 when the MX refuses
the dialogue or does not answer, rc 4 when a tool is missing --
each with a summary line on stderr.

    probe-email <address>

A caller checking many addresses is expected to call this once per
address and bound its own pass; the probe itself caps one dialogue at
20 seconds through hn-timeout. ksh, dig(1), nc(1) and hostname(1)
of the base system and hn-timeout, timeout(1) with a race closed
(sysutils/hn-timeout), are the only tools it needs.

Maintainer: Jean-Gerard Pailloncy <j-g@pailloncy.com>
