It appears that Viktor Dukhovni <ietf-smtp(_at_)ietf(_dot_)org> said:
Postfix has been around since 1997 (alpha), or if you prefer 2001 (1.0 prod).
The SMTP client in Postfix only ever looks at the first digit. I expect
is fairly common. There isn't yet any known response code where we'd expect
the rest to matter. I'd prefer to keep it that way. The remaining digits are
useful for forensics.
qmail which is about as old as postfix looks for 250 and 220 after
EHLO/HELO and STARTTLS but otherwise only looks at the first digit.
I'd flip it around. Do we have examples of MTAs that look at the other digits
in responses to MAIL, RCPT, or DATA comands?
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