When sending an instant message using Skype for Business, you may see the error message "This message wasn't sent to ..." This can be caused for a variety of reasons.
The first user sends an instant message to a second user. The first user sees a message like the screen shot below.
This is due to a bug in the Microsoft Office Software released as an update sometime in October of 2016. The status that the user sees is showing that the second user is signed into Skype for Business Desktop client and a Polycom phone as well.
Ensure both users update their version of Office or Skype for Business to a version that is at least 7766.XXXX or higher. To update Office see How to Manually Update Microsoft Office Products
Note: This resolution is not currently compatible with Mac computers. A compatible resolution is in development but does not have an estimated release date.
The first user sends an instant message to a second user. The first user sees a message like the screen shot below
The second user sees a message on their mobile device like the screen shot below
This is due to the second user being logged into a Skype for Business Mobile Client and the second User ignores the message
At this time, there is not a resolution to this since we are a hosted Skype for Business Solution, Microsoft doesn’t support server side conversation history. If they did support server side conversation history for a hosted solution, then User B who ignored the IM on their mobile device would get a missed conversation notification in their email and User A would not see an error message on their desktop Skype for Business client.
Make certain that users are signed in to a single primary device. If signing in to mobile is necessary, sign out of desktop client.