I've built a few registration systems and always normalize email addresses (remove local part, de-dot gmail addresses) at signup and login.
It helps users who keep trying bobjones@gmail.com when they signed up with bob.jones@gmail.com. Also is pretty good at preventing mass signups using tricks like this.
How do you know where the local part starts? Google uses '+' but nothing stops you from using '-' as a delimiter if you're running your own servers.
Also, how do you deal with spam filters that are designed to spam anything without a local part? Or is this only done to "well-known" domains like gmail.com?
