Mutt/postfix issue

[Henk Koster]

Mail sent via mutt and postfix must appear to have come from my mailbox at
my ISP, H.A.J.Koster at xs4all.nl (as in the header of this message), NOT
from my private "username at localname.localdomain" at home. The domain part
is easily fixed by setting /etc/mailname to "xs4all.nl", but I'm at a
loss to fix the username part. Could anyone help me out here? 

-- 
H.A.J. Koster
"Behavioral axioms are right, but agents make mistakes..."
(attributed to L.J. Savage)