From Quiksoft Corporation:
EasyMail .Net Edition is a set of easy to use, reliable, 100% managed .Net components for creating, sending and retrieving internet email. Full support for SMTP, ESMTP, POP3, IMAP4, SSL, SMIME, MIME, Base64, Uuencode, Quoted-Printable, HTML, SSL, bulk mail, and much more! Supports standard and compact framework. Royalty-free license available. Written in pure C#, EasyMail .Net Edition is fully managed and incredibly fast. It is totally object oriented and flat out the easiest way to add reliable e-mail functionality to your .Net applications.
Version 3.0 features asynchronous sending, direct sending, NTLM authentication, S/MIME (with optional plug-in), and compact framework support.