It should be possible to automatically add new email addresses (which are not yet in public or private address book) when replying an email. This "automatically add to contacts when replying an email" -feature should work with both the webmail and activesync, so it should be handled server side, and not as roundcube webmail plugin.
For example MS Exchange can be configured to do this, and it's useful feature to have. Basicly after you've once replied some person they'll be found in your address book, and it's easier to write to them in the future, or choose them to calendar invitations etc, because there's no need to remember the full email address.
I'm not aware of this being possible with current Kolab 16.1.
It probably should be a configuration option, so one could either disable or enable the said behaviour.