RFC 5321, "Simple Mail Transfer Protocol", October 2008Source of RFC: IETF - NON WORKING GROUP
Area Assignment: app
Errata ID: 4198
Reported By: Jasen Betts
Date Reported: 2014-12-09
Verifier Name: Barry Leiba
Date Verified: 2014-12-09
Section 4.2 says:
Whenever possible, a receiver- SMTP SHOULD test the first digit (severity indication) of the reply code.
It should say:
Whenever possible, a sender- SMTP SHOULD test the first digit (severity indication) of a reply code it receives.
The "sender" is the entity that issues commands and receives response codes (from the "receiver").