In SMTP commands
are sent by the sender
and responses
which all begin with a numeric code
are sent by the receiver
.
but then there's this bit:
RFC5321 sec4.2
Whenever possible, a receiver-
SMTP SHOULD test the first digit (severity indication) of the reply
code.
Huh? Is that an error in the RFC? Do they mean sender
instead of receiver
?
next paragraph they say sender
.
Consequently, a sender-SMTP MUST be prepared to handle codes not
specified in this document and MUST do so by interpreting the first
digit only.
That bit makes sense,