There might be a few reasons for this behaviour, I will list a few fixes here and see if one of them resolves the issue:

Resolution 1:

Delete all the mails in your sent folder.

There are cases where doing this has fixed the issue.

Resolution 2:

Right click on items in your Outbox, click on delete.

If mails don't want to delete, you can Move them to a different folder

Resolution 3:

Adjust Outgoing mail server configuration

Under Tools > Account Settings

Double Click on your email account (Or highlight and click edit)

Where it says: Outgoing mail server - type :

Resolution 4:

Disable Virus scanner

If you have a Virus scanner installed that scans Outlook, it is better to disable it.

Virus scanning can cause issues with Outlooks performance.Windows 8 and 10 has a virus scanner built in.

So there will be no danger to disabling the virus scanner if you have one.

Resolution 5:

Set your Send / Receive time interval to 10 minutes or more.