[felisml]

That's what database-level user IDs are for.

Don't push your technical requirements onto the user when it's not needed.

[jt2190]

If someone uses their work email as a username, they loose their ID if they leave the job. Even worse, the employer may give the same email address to someone else.

[felisml]

If you're allowing emails as a credential reset mechanism, you've already got that problem.