outlook.com MX does not support TLS in 2020.
https://github.com/foxcpp/maddy/issues/301#issuecomment-734923637
Scratch that, it may be a more specific issue. I can observe missing STARTTLS capability only from one location.
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Okay, I discovered that Outlook.com MXes do not advertise STARTTLS capability to blacklisted IPs.
