jstedfast/MailKit — reverse-engineered prompt

Build me a solid cross platform .NET email library for apps and services. I want one package that can send email over SMTP and read mail from IMAP and POP3, with a clean API that feels easy to use in real projects. It should handle secure connections with SSL and TLS, support normal username and password sign in plus modern auth like OAuth2, and work with common providers like Gmail and Exchange.

Please make it reliable for production use, with async support, cancellation, and good error handling. It should support secure auth options, optional proxy support, and the usual mail server features people expect so it works with a wide range of providers. Include clear docs and a few runnable samples that show sending a message, listing folders, reading messages, and downloading from POP3. I also want solid automated tests.

Keep it portable across Windows, Linux, and macOS. If anything is unclear, check the current mail protocol and OAuth docs online and make sensible choices.