I am using migadu [1] and am very satisfied, unlimited domains/users for free or $4 if you want to mail without advertisement below the mails you send.
3 karma · joined October 21, 2016
okay, so you can easily make a rainbow-table and thus your users are not anonymous anymore + if someone has your database and want to know if email@email.com has an account one can easily find that out, even without the need to bruteforce all emails. Btw. in this case it makes absolutely no difference if you or the client computes the hash ;)