SMTPUTF8 address syntax

This document specifies rules for email addresses that are flexible enough to express the addresses typically used with SMTPUTF8, while avoiding confusing or risky elements. This is one of a pair of documents: This is simple to implement, contains only globally viable rules and is intended to be usable for software such an MTA. Its companion defines has more complex rules, takes regional usage into account and aims to allow only addresses that are readable and cut-and-pastable in some community.