Update (April 2023): For Microsoft 365 operated by 21Vianet, TLS1.0 and TLS1.1 for SMTP AUTH will no longer be supported after June 30, 2023. We will be disabling TLS 1.0 and TLS 1.1 usage with SMTP,...
Updated Apr 13, 2023
Version 5.0
Blog Post
3 MIN READ
New opt-in endpoint available for SMTP AUTH clients still needing legacy TLS
Richard170Your circumstance is quite similar to our own, so highly recommend you read DNewb860's response above if stuck. There's a lot more registry keys in addition to just setting up the TLS 1.2 Client/Server. You also have to disable the previous TLS versions and then make sure to update the older .net frameworks so they target 1.2 by default. (I suspect the latter might be what's tripping davies217 up, as they've done everything else up to this point)
https://www.itnota.com/enabling-tls-1-2-default-security-protocol-windows-servers/
Following this article was enough for our servers to get set-up so try this as a starter otherwise DNewb860 had a few additional reg edits beyond this in his comment. Good luck.