[Exim] Envelope-to in user filter files

I have been setting up some .forward files as user filters and have had no
problems checking for values of header variables like $h_to: and $h_from:
but it doesn't seem that I can check the value of $h_envelope-to:

Does the Envelope-to: header get added by exim after it processes the user
filter file? If that's the case, is there any way I can check that same
information in a user filter file?

I'd just check the $h_to: or $local_part but that doesn't always work - the
To: header can contain a mailing list address or something, and $local_part
doesn't have the original address if the message was sent to an email
alias - it has the address the alias points to.

So -- if a message is sent to somelist@??? which is subscribed to by
someone@??? which is really an alias for realperson@???, how can the
user filter reliably get the value someone@??? to check against?

Thanks for any ideas.

--Ryan