<p>I have a project that utilizes the javax.mail.internet.MimeMessage and other related classes that does mime parsing for emails that we receive. This needs to be ported to .NET.</p>

<p>What .Net 3rd party or built in library can I use to replace the Java classes that I'm using?</p>

<p>EDIT: Anything change in the last 9 months since I asked this question?</p>

I've not used javax.mail.internet.MimeMessage, so I can't say how any of this compares, but .NET 2.0 and beyond does have a
System.Net.Mime
namespace which might have something useful for you.
Otherwise, I used
Chilkat MIME .NET
a long time ago and was happy with it.