how can I set the from address in mutt?

[Gary Dale <garydale@rogers.com>]

I want to send e-mail with an attachment via a script. The attachment 
requirement prevents me from using mail but mutt can handle it.

Normally when I send using mail, I can simply use the -r option to set 
the mail user account specified in /etc/exim4/passwd.client. This is 
probably clumsy and there must be a better way of doing it but it works.

Now I want to use mutt to send attachments but it doesn't have the -r 
option. Putting in "my_hdr From:" in .muttrc doesn't do it either.

Can someone suggest a way to make this work? It's probably trivial but I 
can't find the solution via google.