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:
Delete all the mails in your sent folder.
There are cases where doing this has fixed the issue.
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
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
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.
Set your Send / Receive time interval to 10 minutes or more.