FAQ: Security – Secure Socket Layer (SSL) / Transport Layer Security (TLS) has info from the Mono team. Although I don’t know if it’s the same version, you can try mono TlsTest.exe from Duplicati’s utility-scripts. Unless you set up DNS over TLS, I’d guess that the first chance to get a TLS error is with your email server, although that would be more sure if MailKit actually showed up in the errors (or did it?), e.g. as shown here.