For many outlook users mail not sending and getting stuck in the outbox is a very common issue. So, you probably are not the only one looking at this post and it like presently.
1. May be due to issue a large attachment. So, you can check to see if the attachment is not too long.
2. Be sure your send or receive settings are correct
3. If you have no authentication from the mail server so be sure your outlook email account is authenticated
4. The outlook may be scanned by the antivirus you are using. So, you may want to readjust your antivirus settings
5. The outlook may not send messages because if it is in use with another program
6. Messages may not be sent because outlook lost network connection with the server.
If your email is still unsent you should seek more assistance with an expert, you may check other posts online also for further assistance related to this issue. By the way, you can also learn how to fix Outlook Not Sending Emails.
Read More: How to Solve Outlook not receiving emails but can send