I recently published MimeKit , which is much more robust than any other .NET MIME open source XML parser library, and is an order of magnitude faster, and also because it is an actual stream parser and not a recursive descent parser (which also has the added benefit of using less memory).
It has full support for S / MIME v3.2 (including compression, which in fact does not support any of the other libraries that claim to be full support) and OpenPGP.
For SMTP, POP3, and IMAP, you can use my MailKit library, which supports many SASL authentication mechanisms, including XOAUTH2 (used by Google). The SMTP client supports PIPELINING, which can improve mail sending performance, while the IMAP client supports an increasing number of extensions, which also allow clients to optimize their bandwidth.
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