> you cannot know what equivalence rules the mailserver for that domain uses. You're right in general. In my specific post, I do know the equivalence rules, be

> you cannot know what equivalence rules the mailserver for that domain uses. You're right in general. In my specific post, I do know the equivalence rules, because I'm administering the mail system and working with its source code, and the documentation guarantees/requires that internally its email addresses are treated entirely as case-insensitive. What I saw was source code comments about not using `lower(x)` nor Postgres module `citext`, and instead using Unicode case folding ICU and Postgres non-deterministic collations. In the end, what surprised me wasn't about email servers in general, it was about human languages with case folding.