wallace does not implement DSN which make DSN request useless inside Kolab, because postfix will think that because wallace is not able to pass DSN, it is a final SMTP and thus postfix will return success when delivering to wallace.

This has been discuted here : https://kolab.org/hub/topic/248/delivery-status-notification

Bypassing wallace like that solve the issue :

[root@kolab ~]# cd /etc/postfix/
[root@kolab postfix]# diff master.orig master.cf
75c75
<     -o content_filter=smtp-wallace:[127.0.0.1]:10026
---
>     -o content_filter=

It should be possible to implement DSN in wallace:
As far as I understand, the SMTP in wallace (python2.7) implement SMTP but not ESMTP (HELO only and not EHLO and no MAIL FROM extensions) and therefore is not DSN compatible, thus forcing postfix to think it is a delivery SMTP and not a relay one. I've tried to implement ESMTP in python, which should not be that hard given that the same module in python 3 does implement ESMTP although not the DSN extension.