Thanks Steffen, with the very slight alteration of removing the -n (which caused deliver to abort and the message to bounce, oops) from the line I posted, everything works just fine.
For the archives:
dovecot unix - n n - - pipe flags=DRhu
user=user:group argv=/usr/local/libexec/dovecot/deliver -f ${sender} -d
${user}@${nexthop} -m ${extension}
--
This is Art holding a Mirror up to Life. That's why everything is
exactly the wrong way around. --Wyrd Sisters
