I know I'm just not seeing it but how can I go about setting up the following

I have a.com which is the "master" domain of a company that also goes by c.com - I would like to have bob@c.com delivered in the mailbox of bob@a.com automatically without me entering anything other then a rule somewhere that says *@c.com goes to *@a.com

I can't imagine this isn't possible somehow but I'm just not seeing it at the moment.

Thanks

John