When you set up your mail client to receive mail, there are two ways of doing it.
1. This is your default box where all mail wil be delivered at whatever alias the sender wishes to use.
2. Name specific mailboxes where only mail addressed to a specific name is delivered.
When setting up your mail client, in the properties section of the adress you wish to use (using outlook express), there is a field in the servers tab called account name. To just receive all default mail, place your username in this box. To receive name specific mail you need username+name (e.g. mine is didit+mark to receive mail firstname.lastname@example.org).