What you're looking for is the +
. So, for example you could sign up with an e-mail address like:
user.name+foo@example.com
Here, your mail will be delivered to user.name@example.com
, but the "To" field will show the identifier you added after your name as well—in that case, foo
. This is called "sub-addressing".
This trick has been known for quite some time now, an early reference being this LifeHacker article from 2005 where it became popular for the Gmail service which was quite new (and invite-only) at that time.
You can read more about allowed characters in e-mail addresses in RFC 3969 and the actual sub-addressing technique, which is explained in RFC 5233.
Note that this does not work for all mail services. The service has to support this kind of addressing.