Masquerading the from field using sendmail

[M+P]

I have a 4.5 BSD server that does our transactions, like invoices etc. I
would like it to email our customers as the order status changes. I want the
message to appear to come from our sales dept, but sendmail uses the
server's login and server name. I can alter the 'Reply to', but not the
'From' by sendmail options.

I read that masqerade_as can be set in my sendmail config file, and tried to
follow the instructions. My cf directory is empty, so cannot modify and make
the .mc file.

Does anyone know how to go about this? Which .mc file should I modify?  Or
can I just modify the sendmail.cf file?

Thanks,
Mervyn