asterisk users - Feb 2004 - Asterisk and "dial by email?"

I have a friend in australia who I have set an extension up in my asterisk 
server that looks like so:

exten => 1234,1,Dial(SIP/his@email.address)

This works fine.

Now, what I would like to do is make my asterisk server ACCEPT and route 
calls when a user dials say test@test.com.

I already have _sip._udp SRV records set up for the domain I want to accept 
calls for.

I basically want people to be able to call me just via my email address.

How would one do this in asterisk?

Thanks,

John