selecting remotes based on envelope sender

I have (perhaps sketchy) remotes file that works on 1.13 by first
attempting send via one remote, and then when that one rejects based on
enveloper sender, tries the second remote.

This approach doesn't work with 2.1 because nullmailer tries to send a
dsn, but fails, and stops going through remotes at that point.

I guess one approach would be to write a single-remote that checks the
envelope sender and then dispatches to the appropriate remote, but this
seems to be fighting nullmailer pretty hard. Are there better ways to
choose a remote? or can I disable dsn for one remote, on the grounds
that the next one will be tried?

d