[exim] testing transport time rewriting

I just attempted a transport time rewrite that caused a mail loop, so
I'm looking for advice on how to test such things safely.

I believe exim -brw doesn't capture transport rewrites. Though I'm
less sure, I suspect that -N doesn't run the transport at all.

The particular problem was that the rewrite produced a bad sender.
When the mail was refused, this produced a notification message. When
the notification went out, it had a bad sender, and so on.

I tried to rewrite with
*@iron.psg.net $1@${read_file{/etc/mailname}}
but this produced
MAIL FROM:<*@iron.psg.net\n @biostat.ucsf.edu>
and a complaint about an unbalanced <.

Thanks.