Following up the work done at PLANET-4891, we need to exclude aliases that have a + sign, because there is no other way for these accounts to login.
https://planet4.greenpeace.org/handbook/profile/#editing-an-avatar-for-someone-else

Tasks

• Add a check the username part of the user email
• Either limit the scope of the enforcement to emails that have only letter characters for username or exclude the username with a + sign.