RFC5321 “receiver should check […] reply code” (why?)

Question 1

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:

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,

Question 2

Looks like a typo to me. Submit errata.

Answer