What the title says. I’m not out here trying to publish my email to the whole wide world. You should ask for a username, not invent one silently.
Is the idea that folks will take the username and guess @gmail.com, @outlook.com, etc. variants? I guess we can ask users to explicitly set their display name.
That, definitely, also that it might have a real name or other personal info that they might trust nolt with privately but not trust the wide open and searchable internet.