.forward MDA fails, "mail.local: may only be run by the superuser"

Hi,

With the latest snapshot on amd64 (6.6 GENERIC.MP#627), using a "|"-line
in one's ~/.forward makes delivery of mail fail with

Feb 1 08:53:53 pooh smtpd[72575]: d9abac6b3d904e13 smtp connected address=local host=pooh.prefix.duckdns.org
Feb 1 08:53:54 pooh smtpd[72575]: d9abac6b3d904e13 smtp message msgid=8698cb82 size=1824 nrcpt=1 proto=ESMTP
Feb 1 08:53:54 pooh smtpd[72575]: d9abac6b3d904e13 smtp envelope evpid=8698cb8264606654 from=<> to=<kk@pooh.prefix.duckdns.org>
Feb 1 08:53:54 pooh smtpd[72575]: d9abac6b3d904e13 smtp disconnected reason=quit
Feb 1 08:53:54 pooh mail.local: may only be run by the superuser
Feb 1 08:53:54 pooh smtpd[72575]: d9abac6d77a45212 mda delivery evpid=8698cb8264606654 from=<> to=<kk@pooh.prefix.duckdns.org> rcpt=<kk@pooh.prefix.duckdns.org> user=kk delay=0s result=PermFail stat=Error ("mail.local: may only be run by the superuser")
Feb 1 08:53:54 pooh smtpd[19621]: warn: queue: no return path!

The mail is then lost.

I have

pooh % cat .forward
|/usr/local/bin/fdm -a stdin fetch

where "stdin" is a simple mail "account" in fdm(1) that takes messages
from standard input, filters it, and sorts it into the correct Maildir
inbox. For me, this only affects messages originating from the local
system (e.g. crontab output etc., but also messages for root as my root
user is aliased to my ordinary user through /etc/mail/aliases).

I understand that this may well be by design rather than a bug. How
may one use a personal MDA from ~/.forward nowadays, or is that option
completely unsupported from now on?

Regards,

--
Andreas (Kusalananda) Kähäri
SciLifeLab, NBIS, ICM
Uppsala University, Sweden

.