I know that with Gmail they have sub-addressing, which they call plus addressing, and you can enter myaddress+anythingelse@gmail.com and it will be sent to myaddress@gmail.com and tell you it was sent to that address.
Does Yahoo! have anything like this or not?
Sending an email to myaddress+customstring@yahoo.com doesn't work, nor does myaddress.customstring@yahoo.com.