Adding external users to distribution lists

[Alvin]

Hello list,

I'd like to add mailing lists to a Kolab 3.4 server. The mailinglist (Kolab 
distribution group (static)) will contain a mix of users with a local domain, 
but mostly external domains.

Steps I've taken:
- create a list test at example.com
- Add 'contact' users with their mail address as 'External Email Address'.
  'Primary Email Address' remains empty because of an error when trying to add 
the address there: "Email address '....' not in local domain."

Now those users can be added to the list, but sending email to the 
test at example.com distribution group will fail with the message 
<test at example.com>: unknown user: "test"

What am I missing here? Probably those primary email addresses. Is there a 
setting to enable adding addresses that are not in a local domain?