WD20 - using office365 as mailserver

Hi,
one of my clients uses Office365 as their Exchange server.
The address is something like [outlook.office365.com]

Currently my application connects to old-fashion Exchange servers using EmailStartSession()
But with that office365 address it fails with this error: Connection failed: recipient address not found.
Loks like a send-message error, although I am not sending an email yet. Strange...

I also tried EmailStartSMTPSession with no luck.

Can someone tell me how send emails using an Office365 account?