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.
2. Check your mail settings.
Are you sure your mail is supposed to send immediately? If you’ve set a rule to delay message sending, you will see your mail in the outbox for a while.
3. Is your message being slowed down by a large attachment?
Depending on the speed of your internet service provider, if you’ve attached a large file to your email message, you may see it sitting in the outbox because it’s simply taking a while to send, and it may be blocking all subsequent emails from sending.
4. Use the correct password.
If you recently changed your password, you’ll have to make sure Outlook “knows” the correct password. From the File tab, select Account Settings and Account Settings again. Select your account and click Change. Under Logon Information, in the Password box, type your new password.
5. Re-send directly from the outbox.
If your Outlook settings are configured a certain way, you won’t be able to send emails if you’ve opened or clicked on them after you’ve clicked send, i.e. if you’ve marked them as read from the outbox. If this is the case for you, open your email from the outbox, browse to a new folder so you won’t automatically re-mark it as read, and click Send on the stuck email.