[exim] filter "mail" and envelope "from"

For the first time (I think), I'm putting some "mail" commands into an
exim filter file (personal, not system). Until now, I've always used
"deliver". Whenever I do "mail", the envelope sender is empty. This
matters because the place I'm trying to "mail" to discards them (I
reckon because it assumes they are bounce notifications).

Have I overlooked something? I've been thumbing through the Exim book
and the filter spec for a while. I think the envelope sender address is
empty because that's what the autoreply transport does. Is there any
way around that? Or, do others handle this a different way?

BTW, the reason I want to use "mail" instead of "deliver" is because I
want to send only the beginning of the message body.