[horde] Sent e-mails by wrong user

[Toralf Kirsten]

Hi,

I use the Horde 1.2.11 Webmail system running on a small Linux box.
I know that there are newer versions out but the system works over years 
for me to read and manage (move, copy, delete etc.) e-mails online I got 
from other users.
However, whenever I try to send an e-mail, the sender is the user the 
(Apache) web-server is running under (www-data) instead of the login user.
I got this information from the mail log.

│2013-08-07 09:58:26 1V6ydW-0001DA-2q <= www-data at my-domain.de 
U=www-data P=local S=1087 
id=20130807095826.124686gwzxm15gyo at webmail.my-domain.de
│2013-08-07 09:58:27 1V6ydW-0001DA-2q ** user at external-domain.de 
R=smart_route T=remote_smtp: SMTP error from remote mail server after 
MAIL FROM:<www-data at my-domain.de> AUTH=www-data at my-domain.de ...
│2013-08-07 09:58:27 1V6ydX-0001DE-Cl <= <> R=1V6ydW-0001DA-2q U=root 
P=local S=2205 ▒
│2013-08-07 09:58:27 1V6ydW-0001DA-2q Completed

How can I change this behavior?
What options I should set?

Thanks, Toralf