Office365 SMTP requires TLS 1.0

I'm working on a web site hosted in IIS (10) on a Windows server (Server 2019). The site was previously able to send emails out through smtp.office365.com using basic auth. I used IISCrypto to disable TLS 1.0 (and lower) so that only TLS 1.1 and 1.2 were enabled. It caused this error:

The client and server cannot communicate, because they do not possess a common algorithm

Re-enabling TLS 1.0 causes the error to go away. Using SSLLabs.com's server test shows that both my site and smtp.office365.com should be able to support TLS 1.1 and 1.2 with a number of ciphers in common. Why won't they connect?

Solution 1:

That's probably due to .NET framework version is being used.

Reference No.1 https://docs.microsoft.com/en-us/dotnet/framework/network-programming/tls

Reference No.2 https://stackoverflow.com/questions/26742054/the-client-and-server-cannot-communicate-because-they-do-not-possess-a-common-a

As per Microsoft recommendation:

• Target .NET Framework 4.7 or later versions on your apps. Target .NET Framework 4.7.1 or later versions on your WCF apps.

• Do not specify the TLS version. Configure your code to let the OS decide on the TLS version.

• Perform a thorough code audit to verify you're not specifying a TLS or SSL version.