First step: try re-sending
Go to the Send/Receive tab and click Send All. You’ll see a dialog box showing the results of the process, and your email might exit the outbox. If that doesn’t work, move on to the following steps.
1. Make sure Outlook is online.
In your status bar at the bottom of your Outlook message, you’ll see a status of “Disconnected” or “Trying to Connect” which could explain why your messages aren’t sent. If your mail is hosted on an on-premise Exchange server, it’s possible that the local server is down–you may want to check with your admin. If you use Office 365, check your internet connection, wait a few minutes to make sure it’s not a momentary O365 outage, and try again.