Microsoft are famous for ignoring standards and inventing their own so-called standards, and over the years we’ve seen evidence of that in Outlook.
Numerous clients today report that the following applies to Outlook, at least Outlook 2016:
There is no option for STARTTLS for incoming (POP/IMAP) settings. As such, you need to use the older and deprecated option of using port 995 (POP) and 993 (IMAP) over SSL.
