On my IMAP email account I find that most (but not all) of the messages I send are being delayed. I have no rules set up to make this happen.
The message goes into my Outbox and stays there. If I open it from there and look at Options I can see that the Delay Delivery button is highlighted.
If I click on it, it says delivery will be delayed by 10 minutes.
If I un-check the box and press Send again, it still doesn't go.
If I open it and look again, the Delay Delivery button is again highlighted.

My other (POP) email account also managed in Outlook does not do this. Emails send immediately.

How can I solve this problem?