I have reviewed this document as part of the Ops area directorate's ongoing effort to review all IETF documents being processed by the IESG.  These comments were written primarily for the benefit of the Ops area directors.
Document editors and WG chairs should treat these comments just like any other last-call comments.

Summary: 
This document defines a "Limits" extension for the Simple Mail Transfer Protocol (SMTP). 

The document specifies several limits to be registered with IANA. However, I don't see the Limit value being specified. Does it mean that the document simply proposes a new KEYWORD (LIMITS)? 

I am not an expert at SMTP, I have some questions:
- The security consideration says that "a malicious server can use limits to overly constrain clients". Q1: how to prevent clients access malicious server? Q2: how does setting the KEYWORD LIMIT can help this problem 

- Introduction section 6th paragraph says "makes it possible for clients to avoid server errors and the problems they cause. Q: How can setting the LIMITE helps Client avoid Server Errors? 

Thank you, 
Linda Dunbar