RFC 5321, "Simple Mail Transfer Protocol", October 2008Source of RFC: IETF - NON WORKING GROUP
Area Assignment: art
Errata ID: 6561
Publication Format(s) : TEXT
Reported By: Seiichi Hamada
Date Reported: 2021-04-27
Section 4.1.2. says:
Domain = sub-domain *("." sub-domain)
It should say:
Domain = sub-domain *("." sub-domain)[.]
RFC-1034 section 3.1 say " Since a complete domain name ends with the root label, this leads to a printed form which ends in a dot." and also say 'a character string which represents a complete domain name (often called "absolute"). For example, "poneria.ISI.EDU." '.
It is necessary to allow a dot at the end of the domain name.