[Exim] Why does filter "mail" command need transport set by …

My log has:

2002-02-08 14:46:37 16ZJmv-0001Tn-00 == >"Jeremy
C. Reed" <reed@???> <message filter> transporting defer (-1): No
transport set by director

My system filter has:

if $h_subject contains "trying this"
then
mail text "Your message has been received"
endif

The FAQ Q0028 (at the website) says:

Your system filter contains a "save" command, but you have not set
message_filter_file_transport.

My filter doesn't have a "save" command.

The spec (for 3.10) says:

If the system filter generates any deliveries directly to files (via the
save command), a transport to handle them must be specified by setting
message_filter_file_transport, and similar transport options are required
for deliveries to pipes or autoreplies.

"mail" is an autoreply. (But the FAQ doesn't indicate this.)

My exim.conf already has:

address_reply:
driver = autoreply

So I added:
message_filter_reply_transport = address_reply

And it appears to work now.

Can someone update the FAQ Q0028 to properly explain this?

Jeremy C. Reed