Reverse smtpd_sender_login_maps

[Tom Sommer]

I'm trying to get smtpd_sender_login_maps to allow any sasl-auth'd user 
to only send from the domain they are logged in as.

So SASL user "t...@example.com" would be able to send only from 
"@example.com".

So far I have no luck turning the lookup table into this, is it even 
possible?

Thanks
--
Tom