I already knew about the '+' addressing, but not the dots... interesting stuff.
Gmail usernames must be between 6 and 30 characters (inclusive) and made up of only letters, numbers, and dots. The use of dots, however, are optional (that is, Gmail ignores dots when resolving addresses). Google states that "Gmail doesn't recognize dots (.) as characters within a username. This way, you can add and remove dots to your username for desired address variations." For instance, the account email@example.com receives mail sent to firstname.lastname@example.org, email@example.com, etc. Likewise, the account firstname.lastname@example.org receives mail from email@example.com. However, when signing in it is necessary to include any dots used in the creation of the account.
Gmail also supports "plus-addressing" of e-mails. Messages can be sent to addresses in the form: firstname.lastname@example.org where extratext can be any string. Plus-addressing allows users to sign up for different services with different aliases and then easily filter all e-mails from those services. It does not appear, however, that the +string feature works when sending e-mail from a Gmail account to itself (the message so addressed appears only among the "sent" email, not in the "inbox").
The plus addressing is useful for detecting who is responsible for sending you email.
Example - if change my seek.co.nz sign in address to email@example.com then if I ever get email addressed to that address from another company, then its likely that seek sold or passed on my details (not foolproof, but you get the idea).
Also good for sorting and tagging mail.
Other related posts:
Googles online notes application
Gmail reply by chat feature integrates IM and email
Gmail as a free smtp server
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