Gmail Email Address Formatting Issue
Gmail has special rules in place for their email addresses, one of them is that they ignore a 'period' in the email address – they just strip it out when they determine who to deliver it to. So john.j.smith@gmail.com is the same as johnjsmith@gmail.com – to gmail.
So if you run into a scenario where a subscriber tells you they have already unsubscribed, yet they are still receiving emails, be sure to check your all subscribers file for both multiple versions of that subscriber's email address.