I am very late to the party, but today I learned that the good old dotnet SmptClient is considered obsolete and should not be used. Quoting the documentation:

We don’t recommend using the SmtpClient class for new development because SmtpClient doesn’t support many modern protocols. Use MailKit or other libraries instead. (source)

Interestingly, Microsoft is recommending a third-party open-source library as an alternative. I hope we’ll see more of that in the future.

I just finished integrating MailKit in our backend. I must say that I’m pleasantly surprised by its rich feature-set and the elegant and straightforward design, which makes getting on-board super easy. It’s built on top of the excellent MimeKit, after all, and authored by the very same author Jeffrey Stedfast.