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 to use to the External Addresses field in the Kolab Web Admin.

If you also want to allow users to choose external addresses for identities in the Kolab Webclient, you have to edit /etc/roundcubemail/kolab_auth.inc.php and find the $config['kolab_auth_addressbook'] array. In it, there is a fieldmap where you add

'email:external'    => 'mailalternateaddress',

to it. That’s all! Now your users should also see their external email addresses in the drop down list when editing their identities in Roundcube.

Should setting up your own Kolab server be too much of a hassle for you, you can also get an account from MyKolab and support Kolab’s development this way.