When it comes to email address normalization, it sounds like we could do with a standardized way for a domain to express normalization policies.

Could be as simple as publishing a set of regular expression subsitution rules, specifying (for example):

* render to lower case (because this particular domain is case insensitive)

* drop periods (because this domain treats them like gmail does)

* drop '+' and any subsequent characters (because this domain treats them like gmail does)

* ASCII only (because mail software is old, and doesn't support unicode)

Etc.

Each domain could then publish their own rule, perhaps in a DNS txt record, and anyone needing to check if two email addresses alias to the same could run the correct checks.