Google lets you put any number of periods in your gmail address, and it ignores all of them when receiving mail for you.
Try it; it's fun!
Spammers have taken advantage of this, unfortunately, to spam more and more easily.
If you see a Gmail address with about as many periods as letters, it is probably a spammer. If that address turns up twice, differing only by the number or placement of periods, it is pretty certainly a spammer.
This module does not automatically block registration with such addresses, but once you block one address, it will block any more from registering.