The e-mail icon is showing next to the name of deleted users in old postings they made in the past. You can go through the process of sending an e-mail to the deleted user by clicking on the icon but of course it will not be delivered as the details of the user are no longer in the database.

In the past the e-mail icon did not show if the user had been deleted and I have only just noticed that it now shows so it may only be specific to the lastest version of the forum.