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MailKit C# SmtpClient.Connect() to Office 365 generating exception: "An existing connection was forcibly closed by the remote host"

Got another error after posting this question which led me to the answer:

Handshake failed due to an unexpected packet format

The solution is to connect to Office 365 like this instead:

smtpClient.Connect("smtp.office365.com", 587, SecureSocketOptions.StartTls);