It happens to us in the following way:
An administrator puts a user in the status: Email invalid (bounce). The user uses a hotmail email address.
Email is banned ( *@hotmail.*).
The user logs in again and returns to using the same hotmail email address.
In theory, he should not be able to use any hotmail addresses again, as they are banned.
I just did a test and this is indeed happening.
If the administrator bans a user's email and then that user is put in Email invalid (bounce) status, the user has the ability to keep their email even though they are banned by simply entering their password.