Added to Git and CVS.

Changes have been made in CVS for this ticket:

http://cvs.horde.org/diff.php/imp/docs/CHANGES?rt=horde&r1=1.699.2.373&r2=1.699.2.374&ty=u
http://cvs.horde.org/diff.php/imp/lib/MIME/Viewer/itip.php?rt=horde&r1=1.37.2.43&r2=1.37.2.44&ty=u

This did not fix the issue for me as the current username and 
credential were not transferred with the parameters. 
imp/lib/Compose.php does add the current user and the password to the 
parameters before sending mail.

The following patch fixed the issue for me:

http://hg.pardus.de/cgi-bin/hg.cgi/horde/HORDE/file/0f8c1b39f7eb/imp/HK-GW-Auth_for_invitations.patch

Fixed for IMP 4.3.1.

Changes have been made in CVS for this ticket:

http://cvs.horde.org/diff.php/imp/lib/MIME/Viewer/itip.php?r1=1.37.2.41&r2=1.37.2.42&ty=u

I have configured Horde to use SMTP AUTH for submitting messages 
through 'config/conf.php':

     $conf['mailer']['params']['port'] = 587;
     $conf['mailer']['params']['auth'] = true;
     $conf['mailer']['type'] = 'smtp';

This works just fine when writing new messages or sending replies:

Sep 24 16:37:57 mail postfix/smtpd[19746]: connect from localhost[127.0.0.1]
Sep 24 16:37:57 mail postfix/smtpd[19746]: setting up TLS connection 
from localhost[127.0.0.1]
Sep 24 16:37:58 mail postfix/smtpd[19746]: Anonymous TLS connection 
established from localhost[127.0.0.1]: TLSv1 with cipher DHE-RSA-AES256
Sep 24 16:37:58 mail postfix/smtpd[19746]: 49B7A6041C: 
client=localhost[127.0.0.1], sasl_method=LOGIN, 
sasl_username=user@example.com
Sep 24 16:37:58 mail postfix/smtpd[19746]: disconnect from 
localhost[127.0.0.1]

The exception to this rule is, when I want to accept/deny an 
invitation that was send through Kronolith. After hitting the 'Go' 
button, the following error message appears:

"authentication failure [SMTP: Invalid response code received from 
server (code: 535, response: 5.7.8 Error: authentication failed: 
generic failure)]."

In the mailserver logs is logged at the same time:

Sep 24 19:09:28 mail postfix/smtpd[21873]: connect from localhost[127.0.0.1]
Sep 24 19:09:28 mail postfix/smtpd[21873]: setting up TLS connection 
from localhost[127.0.0.1]
Sep 24 19:09:29 mail postfix/smtpd[21873]: Anonymous TLS connection 
established from localhost[127.0.0.1]: TLSv1 with cipher DHE-RSA-AES256
Sep 24 19:09:29 mail postfix/smtpd[21873]: warning: SASL 
authentication failure: All-whitespace username.
Sep 24 19:09:29 mail postfix/smtpd[21873]: warning: 
localhost[127.0.0.1]: SASL LOGIN authentication failed: generic failure
Sep 24 19:09:29 mail postfix/smtpd[21873]: disconnect from 
localhost[127.0.0.1]

Accepting messages works just fine when I use the build in sendmail 
binary, but not when the replies go through SMTP AUTH. I would like to 
use SMTP AUTH, so that I don't have to duplicate the existing 
'smtpd_sender_login_maps' that prevent From address spoofing.