[ale] sendmail setup help

[Michael D. Hirsch]

I have a system that uses sendmail.  I'm trying to do 2 things:

1. local users (e.g. usera) are delivered to our mail server with 
@nubridges.com apended (e.g. usera at nubridges.com).  

2. forward particular users (userb) to another address 
(userc at anotherplace.com).

I know how to use /etc/aliases to accomplish 2, but I can't figure out 1.  It 
looks so easy.

Anyone know the right incantation for the sendmail.mc file?

Thanks,

Michael