“There are three common reasons why Gmail users think they’re receiving someone else’s mail.
Reason one:Your address is similar but has more or fewer dots (.) or different capitalization.
Sometimes you may receive a message sent to an address that looks like yours but has a different number or arrangement of periods. While we know it might be unnerving if you think someone else’s mail is being routed to your account, don’t worry: both of these addresses are yours.
Gmail doesn’t recognize dots as characters within usernames, you can add or remove the dots from a Gmail address without changing the actual destination address; they’ll all go to your inbox, and only yours. In short:
homerjsimpson@gmail.com = hom.er.j.sim.ps.on@gmail.com
homerjsimpson@gmail.com = HOMERJSIMPSON@gmail.com
homerjsimpson@gmail.com = Homer.J.Simpson@gmail.com
All these addresses belong to the same person. You can see this if you try to sign in with your username, but adding or removing a dot from it. You’ll still go to your account.
If you get mail that seems to be intended for someone else, it’s likely that the sender entered the wrong address, just like if you’ve ever dialed a wrong phone number for someone. In these cases, we suggest contacting the original sender or website when possible to alert them to the mistake.
One last thing: Google Apps does recognize dots. If you’d like to have a dot in your username, please ask your domain administrator to add your preferred username as a nickname.”

Now, this doesn’t mean that people aren’t exploiting this, so take care.