Troubles sending as alias from Outlook2016

Hi Forum
Just set up a Zimbra server with Zextras Mobile.
On one account I have 4 aliases - which I would like to be able to send mail as from outlook.
In the zimlet desktop and allso from web is this possible - but I cannot see the oiption from Outlook.

Is there anything I'm missing here ? which should be configured from Outlook/Zextras Mobile for this option --> Send as are available

THnaks in advance