Unfortunately our users/aliases database is case sensitive. (Perhaps I should have defined the columns with COLLATE NOCASE, see https://www.sqlite.org/datatype3.html.) Postfix always queries the tables in lowecase, so mail delivery would fail if a user or alias were defined with any capital letters. It would have also been possible to add multiple euqivalent addresses into the database with different case.

This commit rejects new mail users that have capital letters and forces new aliases to lowecase. I prefer to reject rather than casefold user accounts so that the login credentials the user gave are exactly what goes into the database.

https://discourse.mailinabox.email/t/recipient-address-rejected-user-unknown-in-virtual-mailbox-table/512/4