This is called a "catch all"

Mapping is done using /etc/postfix/virtual file.

vim /etc/postfix/virtual
Append code as follows, replacing domain and emailusername with actual values:

@yourdomain.com emailusername
Save and close the file. Run following command:

postmap /etc/postfix/virtual
Also make sure you have following line in /etc/postfix/main.cf file:

virtual_alias_maps = hash:/etc/postfix/virtual
If you just added above, line reload postfix:

service postfix reload
From http://www.cyberciti.biz/faq/howto-setup-postfix-catch-all-email-accounts/