@MarkLL I just wanted to point your attention to:
Office 365 SMTP - 535 5.7.3 Authentication unsuccessful - #12 by d3mad
Lubos has changed the underlying framework of .Net within the binary itself by including .Net Core as part of the binary instead of bundling a framework that had the TLS issue.
I have tested this latest implementation and the .Net issues with TLS appear to be fixed. I have confirmed it on a box that previously defaulted to v1 and it now defaults to v1.2. My mail server wasn’t rejecting TLSv1, but I’m confident this latest version will fix the issue you (and possibly others) were having.
Cheers