If you have an email address say firstname.lastname@example.org, then you will get all the mails to the mail addresses which is containing . between your email address.
read it from Google
"Because Gmail doesn't recognize dots as characters within usernames, adding or removing dots from a Gmail address won’t change the actual destination address. Messages sent to email@example.com, firstname.lastname@example.org and email@example.com are all delivered to your inbox, and only yours." For security reasons, when you log in to Gmail, you must enter any dots that were originally defined as part of your username.