Thank you for responding.
isValidEmailAddr is what I'm using currently. [1] But using the hook (1) does not show helpful message to user, It just says “your email address is not confirmed” for some reason. (2) The hook is invoked too many times than I expected, for example it happens whenever a user open an editing page.
But thank you again.
1: https://github.com/femiwiki/UnifiedExtensionForFemiwiki/blob/7ba08a3bd8a2f56...