By default, Kolab does not allow you do send mails from external email addresses that are not hosted by Kolab itself. However, it is quite easy to change this.
In /etc/kolab/kolab.conf find the section [kolab_smtp_access_policy] and add mailalternateaddress to the address_search_attrs. Now you just have to enter all external addresses a user should be able . . . → Read More: Using External Email Addresses with Kolab