A couple of times now we’ve needed to forward foo@oursite.com to my@gmail.com. If you’re running postfix here’s how to do that.

nano /etc/postfix/virtual

Add

from@email.com to@email.com

then run

postmap /etc/postfix/virtual

If the above doesn’t work, make sure you have the following line in /etc/postfix/main.cf:

virtual_alias_maps = hash:/etc/postfix/virtual

and type

service postfix reload

Thanks to Matt Simmons of ServerFault for his answer.