at

http://community.discountasp.net/showthread.php?t=3762

there's a useful discussion that shows one can have a message with just a text/plain AlternateView, so they can set TransferEncoding there (modified their sample a bit since they were instantiating SmtpClient class twice by accident)

MailMessage email = new MailMessage(SENDER_EMAIL, RECIPIENT_EMAIL);

email.Subject = SUBJECT;

AlternateView plainView = AlternateView.CreateAlternateViewFromString(sMessageBody, Nothing, "text/plain");

plainView.TransferEncoding = System.Net.Mime.TransferEncoding.SevenBit;

email.AlternateViews.Add(plainView);

SmtpClient _client;

this._client = new SmtpClient(SMTP_SERVER, 25);

this._client.Credentials = new NetworkCredential(CRED_ID, CRED_LOGIN);

this._client.Timeout = TIMEOUT;

this._client.Send(email);



There's a CreateAlternateViewFromString that takes an encoding param (the example I used above had Nothing for the encoding)

http://msdn.microsoft.com/en-us/library/ms144596.aspx

so one can add an AlternateView of "text/plain" with the encoding the user asked and then tell it to use "base64" TransferEncoding (in the example I copy/pasted above they were using 7-bit)

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Also in the same way can add more AlternateViews for HTML mimetype ("text/html" I think is the MIME type, but better check it out). For HTML with no images/attachments (just formatted text) it's easy to add with few changes I suppose