[exim] wrong senderaddress used in mail transport process

Hi guys,

i think i have a small problem with my exim setup :

Here's the SMTP Protokoll ( slightly disguised )

220 c1.resellerdesktop.de
ESMTP Exim 4.76 Fri, 28 Oct 2011 16:59:10 +0200
250-c1.resellerdesktop.de Hello p548509f5.dip0.t-ipconnect.de [84.133.9.245]
250-SIZE 52428800
250-PIPELINING
250-AUTH PLAIN
250-STARTTLS
250 HELP
AUTH PLAIN XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
235 Authentication succeeded
MAIL FROM:<source@???> SIZE=552
250 OK
RCPT TO:<destination@???>
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
....

As you can see, there is no username involved, besides the auth plain
part, but the username is used as FROM Part inside exim :

2011-10-28 16:59:10 1RJntm-0002MI-Pp <= username@???
H=p548509f5.dip0.t-ipconnect.de ([192.168.0.32]) [84.133.9.245] P=esmtpa
A=plain:resellerdesktopdeuser1 S=892 id=4EAAC33A.7000501@???
2011-10-28 16:59:11 1RJntm-0002MI-Pp => destination@???
R=dnslookup T=remote_smtp H=mail.mailaddress.de [212.162.x.x]
X=TLSv1:AES256-SHA:256
2011-10-28 16:59:11 1RJntm-0002MI-Pp Completed

If the delivery failes, username@??? is used to send
the deliverymsg to. This will fail as the mailserver does not accept
user@server adresses , because it won't find them in the database.

Which option did i sent wrong to force exim to use the
userauthcredentials as MAIL FROM ????

best regards,

M:Schwarz