STARTTLS is a way to take an existing insecure connection and upgrade it to a secure connection using SSL/TLS.
To change your existing connection to TLS in your Outlook, please use the follow guide.
Kindly note that this guide uses Outlook 2007 and newer Outlook version, the location of the mentioned function may be different.
Before proceeding the following, kindly note down of the current working value for the "Outgoing server (SMTP)" and the down drop selection for "Use the following types of encrypted connection". This is so that in the event there is any issue afterwards, you may revert back to the working value and settings.
Click on "Advanced" tab. Under "Outgoing Server" > "Use the following types of encrypted connection" click on the drop down and select "TLS".
Outgoing port able to use with TLS are: 25 and 587.