A lot. To send email, you can use the built-in SMTP support. But
posting to a newsgroup requires communication with an NNTP server. The
NNTP protocol is completely independent of SMTP, and there's not any
support for NNTP built-in to .NET.
I suppose you might be able to find an SMTP server somewhere that will
automatically forward email messages to a newsgroup of your choosing.
But that's a server configuration issue, not a C# or .NET question.
Pete