This helps ensure the sender does not get the following error: “We couldn't send this message.” After 5 minutes of any activity, the Mac client does not auto-accept the messages to ensure mobile endpoints are able to accept incoming messages as necessary. If server-side conversation history is turned off in your organization, by default, the Skype for Business on Mac client will auto-accept incoming messages if the message arrives within 5 minutes of the Mac client being active. Admin: How do you increase the auto-accept period on Mac client?