[Exim] return-path user -- local vs. smtp

I've searched the archives but wasn't able to find the answer to my
question. Using kmail, if I send a message using /usr/sbin/exim, it comes
through with the return-path username altered to match my login name
(kurtl) instead of my email address (kurt).

If I simply change the transport to SMTP and use localhost as the server, it
comes through unaltered -- just like I want it.

So, my question: what is the magic setting that I need to alter to get this
to work locally? I tried the -f option, but kmail complains if I set it to
use '/usr/sbin/exim -f', presumably because I'm not also passing it an
email address. However, since I use several different email addresses,
passing -f the correct email address all the time would be difficult.

Thanks.

--kurt