There is no method in System.Net.Mail that allows you to do this. You'll have to write a parser and then build up the mail message manually. It's not that hard to do, though. Here's the structure of an email:
Fist you'll have headers that look like HEADER-NAME : CONTENT and a header will end with a /r/n that is NOT followed by a whitespace character. If a header has /r/n it means that the headers' value is continued on the next line. For Content-Transfer-Encoding, you'll need to potentially decode the byte encoding (quoted-printable or base64) in the message body. After the headers, you'll have the email body itself which is separated from the headers by an extra line (\r\n\r\n). This may just be text or HTML or something, in which case just parst it literally and decode it as per the transfer encoding. The quoted-printable encoding for the body is slightly different from the headers so read up on it in RFC 2045, 2046, and 2047. Base64 encoded content will have a newline character between lines but otherwise isn't anything special.
If you have attachments and stuff then you have to be a bit more clever. These are usually after the body and will have a Content-Type header that will tell you how to decode the attachment and some info about it as well as possibly a Content-Disposition header that provides some metadata about the attachment. Attachments are usually based64 encoded.
Finally, if you see a multipart/mixed under the message's content-type header, it means that you need to parse each part individually. It will give you a string of chars that denote the boundary between parts. Each part will then have a Content-Type of its own that says what it is as well as other headers that help describe that part. You'll have to parse out each of these parts and then decode them manually, writing attachments either to in-memory streams or to a file.
Once you have all of this, you can then re-create the email in code. There has been some talk on the NCL team of addressing this need somehow but nothing has been decided and I can't promise anything.
As an example, here's what a .eml file with some custom headres, two alternate views (one text, one html), a linked-resource for the HTML view that's an image, and a .txt attachment will look like. I've included some unicode so that you can see the encoding mechanisms. The example companies, organizations, products, domain names, e-mail addresses, logos, people, places, and events depicted herein are fictitious. No association with any real company, organization, product, domain name, email address, logo, person, places, or events is intended or should be inferred.
X-Sender: [email protected]
X-Receiver: "Dave Barnett"
[email protected]
X-SalesAgent: =?utf-8?B?VG9sbGV2c2VuLCBCasO4cm4=?=
MIME-Version: 1.0
From: [email protected]
To: "Dave Barnett"
[email protected]
Reply-To: =?utf-8?Q?Tollevsen,_Bj=C3=B8rn?=
[email protected]
X-Priority: 5
Priority: non-urgent
Importance: low
Date: 28 Aug 2009 13:03:40 -0700
Subject: order details
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Content-Type: multipart/alternative; boundary=--boundary_0_61ec80d2-1bf7-4554-922e-634e8e8a1067
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Content-Type: multipart/related; type="text/html"; boundary=--boundary_1_e1f66bde-4086-4084-ad21-245b55a3d7a0
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Content-Transfer-Encoding: base64
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Thank you for your order.
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Aug 2009 13:03:40 -0700"
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