[Exim] Changing/adding return_path

Hi,
I need to know how to configure exim to reflect my true pop3 address form my
ISP and not my machines domain and user. (I have been getting mail returned
with the error: "senders address must exist.") I have read that you can add
the variables return_path and sender_address, but can't figure out the
syntax or where to apply them. I added the line: my_hdr From: \"$realname\"
\<me@???\ > along with the set realname variable to my .muttrc file,
but this didn't do the trick. My return path still says user@mymachine.
I'm running debian 2.1 with exim, fetchmail, and mutt with a dialup ISP. I
have successfully sent and received mail, but obviously from domains who
don't care who the sender is.
Maybe I'm not even headed in the right direction, I'm still figuring this
out. If anyone could point me in the right direction, I'd appreciate it.

Thank you,
--Marc