How do I check which version of TLS is being used in Outlook 2010 with Office 365 Exchange Online mail server?
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I think it should be TLS 1.2, because as of October 31, 2018, Office 365 will no longer support TLS 1.0 and 1.1. Preparing to use TLS 1.2 in Office 365 https://support.microsoft.com/en-us/help/4057306/preparing-for-tls-1-2-in-office-365
You could use Network Monitor to check which version of TLS is being used in Outlook 2010 with Office 365. https://www.lee-ford.co.uk/psa-tls1-2-ticking-timebomb/