If you have a known email client that is being used, and it supports  
automation through .NET (e.g. Microsoft Outlook), then you can use the  
automation API to access email messages.

Otherwise, there's no email-specific support in .NET for reading email.  
You'd need to write your own.

POP and IMAP are reasonably simple and well-documented APIs, so if the  
protocol you're dealing with is either of those it probably wouldn't be  
too hard.  You'll want to find a form more specifically related to mail  
clients using those formats though, except of course for questions that  
really are about C# and/or .NET and only tangentially related to the fact  
that you're trying to implement a mail client.

Pete