reject outgoiong mail not from domain of host

[John G. Heim]

I want to make sure that any messages my system sends out always has a 
sender of someb...@math.wisc.edu. I am looking at configuring the 
smtpd_sender_restrictions directive.

smtpd_sender_restrictions = 
hash:/etc/postfix/all_addresses_at_math_wisc_edu,reject

Am I close?