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 : mail.igneous.co.za
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.