https://github.com/phpbb/phpbb/blob/master/phpBB/includes/functions_user.php#L206

First read https://stackoverflow.com/a/9808332/1833322

Basically why this is not a good idea to go to lower case:

1. it's against e-mail spec (RFC)
2. User does not see back his e-mail in the way he/she entered
3. Unchanged e-mail address always works as intended. To lower case doesn't work on some e-mail systems (for example MS Exchange as in stackoverflow comments)