Since .NET Core 1.0 does not include System.Net.Mail, developers now have the option of at least using MimeKit and MailKit as a way of sending mail in their applications.
MimeKit is a fully managed and open source MIME parser library for .NET that is as fast as (or faster in some cases!) any native parser written in C++. Also included is support for DKIM signatures, S/MIME v3 and OpenPGP.
MailKit is a fully managed and open source email library with support for SMTP, POP3 and IMAP.
API documentation can be found at http://mimekit.net/docs