[exim] Empty sender from authenticated users MUA

Hi,

in few words: should MUAs be allowed to set an empty sender (or what
else is happening here)?

I sometimes see an empty sender comming in from authenticated users
MUA like this in my log:

2007-11-22 10:45:02 1Iv8cH-0000Xc-Ps <= <> H=p57b2e2c9.dip.t-dialin.net
(Bro) [87.178.226.201] P=esmtpsa X=TLSv1:RC4-MD5:128
A=auth_login:localuser@??? S=2663

2007-11-22 10:45:02 1Iv8cH-0000Xc-Ps => externaluser@???
R=dns_lookup T=remote_smtp H=mailin.rzone.de [81.169.145.100]

2007-11-22 10:45:02 1Iv8cH-0000Xc-Ps Completed


Because the users are authenticated, the messages are handled in
submission mode, with the $authenticated_id being the qualified address
of the user:

  accept  authenticated = *
          control       = submission/domain=

The docs say, submission mode fixes From: and adds Sender: Headers.

But what will the envelope sender look like to the external MTA? Will
this be sent as if it was a bounce message? And does this have to be
permitted for some reason or should this be blocked?

What I know is, the users where I see this use Outlook or Outlook
express and most of their messages show the correct sender address in
the logs.

Thanks for any hint.

--
Andreas