qmail-smtp and error messages

[Markus Stumpf]

If I get the error message e.g.
    503 RCPT first (#5.5.1)
what does the "(#5.5.1)" mean? I thought I'd read somewhere it refers
to the section 5.5.1 in RFC 1123, but, I can't find such a section in
RFC 1123 (neither in RFC 821).
I've grep'ed through my private qmail archive, searched the list
archives, read the man pages and the FAQs, looked at qmail.org
but I can't find the reference any more :(

Someone please enlighten me (and put the info on qmail.org?)

Thanks

        \Maex