I have one more question:
FRC 1869 defines 2 new SMTP reply codes in section 6.1:
If the server SMTP does not recognize or cannot implement one or more
of the parameters associated with a particular MAIL FROM or RCPT TO
command, it will return code 555.
If for some reason the server is temporarily unable to accomodate one
or more of the parameters associated with a MAIL FROM or RCPT TO
command, and if the definition of the specific parameter does not
mandate the use of another code, it should return code 455.
They are not in RFC 2821 which obsoleted RFC 1869. Bug or feature?