Gmail and other email services allow users to extend their address using the + (plus) character.
For example, mail to
spamme@gmail.com
spamme+sideways@gmail.com
spamme+tomorrow@gmail.com
will all be receivable by the owner of spamme@gmail.com
In its default configuration, Drupal 7 would allow the owner of spamme@gmail.com to register 3 separate accounts using the 3 addresses above... and more if they want!
I would like to prevent this type of multiple registration.
While I could just create a rule that blacklists email addresses containing the + character, there are legitimate uses of this.
I would prefer to be able to check for other equivalent email addresses, where in the above example, all 3 addresses are equivalent, because they are all collectable by spamme@gmail.com, and allow only one of them.
Is there some way to do that?