Kopano works fine with webapp, but using an iphone I already get an error with a 300 k attachment:
E-Mails können nicht gesendet werden, Die Nachricht wurde vom Server abgelehnt, da sie zu groß ist.
the max message size in Postfix is only part of the solution here, what you are running into is most likely memory limits within php (you could verify this when looking at the error log of the Apache webserver on UCS). The reason why it works with WebApp is because WebApp brings a .htaccess file that overrides the defaults.
You need to adapt your php configuration to use the following values: