-f flag for sendmail

[Andii Jursich]

Last night I had a few routers go down, i was a sleep so I didn't get any of
the notifications, needless to say it escalated and sent the messages to my
cell phone.  I would have gotten these but cingular didn't like the domain
it came from, it returned them error 552 or 553 I don't remember.  I figured
out that I need to add the -f option to the sendmail line so I can have it
send from a valid domain.  My question is where should I add the -f flag?
 
Thanx,
Andii