The following fault is present in Outlook 2016 for Mac Version 15.26.1 (160916) when using Full Screen Mode in macOS Sierra.
You are using Outlook 2016 for Mac Version 15.26.1 (160916)
You have macOS Sierra 10.12 installed
Outlook 2016 for Mac is running in Full Screen Mode
When running Outlook 2016 for Mac in Full Screen Mode you double click on an email in your inbox. The screen dims to focus on the email you are viewing. If you click on reply the screen dims completely. You can still type in the reply email but when you go to click on send the email minimizes to the bottom of the screen. If you then click on the minimized message you can then click on the send button.
Upgrade Outlook 2016 for Mac to Version 15.27