Users can't communicate with external contacts

Problem

You experience one of the following symptoms in Skype for Business Online (formerly Lync Online):

• Instant messages aren't delivered to external contacts. When you try to send an instant message to an external contact in Skype for Business Online, you receive the following error message:

  This message was not delivered to joe@contoso-federated.com because the address is outside your organization and is not federated with your company, or the address is incorrect. Please contact your support team with this information.

  
  Note In this error message, joe@contoso-federated.com is a placeholder.The following error message may also be recorded in the Lync 2010 UCCAPI logs:

  403 Forbidden: From URI not authorized to communicate with federated partners.

• Presence is unknown for an external contact. When you search for the external contact, or you review an external contact who's already added to your contact list, the presence icon is unavailable (it appears dimmed), and Presence Unknown is displayed.
• External contacts can't join Skype for Business Online meetings. When the external contact receives a Skype for Business Online meeting invitation and then clicks Join Online Meeting, the user receives an error message that states that the person doesn't have permission to join the meeting, and the connection fails.

Solution

The first step to resolve issues that affect Skype for Business Online is to scope the issue appropriately. The following table will help determine what the most likely cause of the issue is, depending on the type and number of contacts that are affected.