[Exim] filter "mail" command generating bogus RCPT TO:

I must be doing something wrong here. I've just searched the list for the last
hour and found multiple references to the filter "mail" command working for
other people. But for some reason, when my system filter says...

if $header_from: contains "me@???"
then
mail to test@??? text "this is a test"
finish
endif

I get this in my log...

** >test@??? <system-filter> T=filter_smtp: SMTP error from remote
mailer after RCPT TO:<>test@???>: host mail.mydomain.com
[200.200.200.222]: 501 Syntax error, parameters in command "RCPT
TO:<>user@???>" unrecognized or missing

This is with

system_filter_reply_transport = filter_smtp
system_filter_user = exim

set in my configuration file. The only thing I can think of is in the
definition of my system_filter_reply_transport, which is just:

filter_smtp:
driver = smtp
hosts = mail.mydomain.com

The error message in the log shows one extra '>' in the RCPT TO: line exim
generates (the one right before test@...). It uses the test@???
address and the hosts from my system_filter_reply_transport. And it used to
give me a trusted_user error until I set "system_filter_user." So I can't
figure out what I'm still missing.

Any suggestions would be greatly appreciated.
Thanks!

--
Rick Ennis
ennis@???
mobile: 617-899-0744