case sensitive email account for aknowledgement

[LPailles]

I've added a collaborator John Doe John.Doe@mycompany.com
He read and aknowledged as john.doe@mycompany.com

Now he's still in validation pending state as John.Doe@mycompany.com and I have a warning that a user john.doe@mycompany.com confirmed reading but was not in the llist of pending users.