Delay rule in Outlook causing emails to get permanently stuck in outbox

I have a three minute delay rule set up in Outlook. Previously this has worked with no issues, however, now about 50% of my emails get stuck in my outbox when they send. After hitting send the email goes to the outbox with the 'editing' symbol (piece of paper with pencil) next to it rather than the 'send' arrow symbol. If, after three minutes, send/receive is pressed the email won't send. If the email is opened and send hit again it wont send. The only way to send the email is to copy the text into a new email and send that. If I turn the rule off the issue goes away.

My internal IT department has been unable to help, they have run the repair function, created a new profile and reduced the downloaded emails, none of which helped.

Thanks in advance for contributions.