@sampo2910 of None is the only option that works, then that's probably the only one your email provider supports. It should be fine as long as you don't need your emails to be perfectly secure.
When users sign up using Facebook SSO , Facebook creates a random-numbered email address for them such as [email protected]
I think a similar problem was already reported here: https://community.nodebb.org/topic/5985/facebook-sso-plugin-email-issue
However, all emails sent to these kinds of addresses are returning back to us. This was already happening, but it was not bothering us so far unfortunately, recently we started to have a lot of "Delivery Status Notification (Failure)" emails... so, we would like to solve this.
And on the Facebook postmaster page, we can confirm the problem as "RCP-P1 The attempted recipient address does not exist."
Since the forum sees these accounts as valid and activated, it keeps trying to send an email at any action that causes email delivery. And of course, all emails are being failed to be delivered and returned back to us.