Duplicated emails in Mail (Outlook) Sent folder

Hi,

When I first set up my new MacBook Air (Retina, 13 inch 2018), just 2 weeks ago I distinctly remember checking an option to store sent mail items on the server.

Since doing that however, all mail sent from my Mac is duplicated in the Sent folder of the Mail app. The sent mail is also duplicated in the Sent folder of all my other devices (iPhone X, iPad, iTouch).

However, if I send an email from any of the before mentioned devices (excluding the Mac), the sent mail is not duplicated.

After doing a significant amount of online research it appears the fix would be an easy one, if the option I needed to uncheck could be found anywhere.

What I'm trying to do is the following:

1. Launch Mail app

2. Select the Mail preferences from the Mail Menu Bar

3. Click the Accounts tab

4. Highlight Outlook account

5. Click Mailbox Behaviours

6. Uncheck "Store sent messages on the server"

7. Close the Mail preferences and send a test message

However, Step 6 (the most important step) cannot be done as "Store sent messages on the server" does not exist. See attached image.

In speaking directly with an Apple Support consultant, he was surprised this option was no longer available and could not offer a solution.

But this irritating issue really needs to be resolved.

Can you please advise how I can stop messages being stored on the server so that they are no longer duplicated.

FYI, I've deleted and re-added my Outlook IMAP email account to no avail and my iCloud IMAP account is not activated.

Thanks