Outlook was not encrypting email connections, even though SSL/TLS was clearly enabled in the account settings. It looks like, that bug dates back to at least Outlook 2007:
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Indeed! Twenty years is wild! 
Makes you wonder how many “encrypted” connections out there are quietly falling back to plaintext without anyone noticing. The settings said one thing and the wire said another… worst kind of bug. 
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I am afraid, the poster is wrong. Frankly, that is not Outlook bug at all. That feature is known as opportunistic TLS or opt-in encryption.
Namely, POP session begins unencrypted via port 110 and after starttls command issued by client traffic becomes encrypted, provided that server supports it. Without server support traffic remains unencrypted.
“Enable SSL/TLS” checkbox is optional. It is not an explicit switch. “Enable SSL/TLS” combined with explicit POP port 110 allows opportunistic TLS if any. That is Outlook developers made no error.
Obviously Dovecot does not support opportunistic TLS or its implementation is incompatible with Outlook. 
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Looks like it’s from this article:
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