Answered: MailMessage.Encrypt, problems with german umlauts like äöü

I guess that a problem in the e-mail message such as a mismatch between MIME entity's charset and HTML charset is more likely to be a cause than a bug in Outlook Web Access. How do you actually construct the message? Do you assign the HTML text via MailMessage.BodyHtml, or via MailMessage.Views?

One random tip that might help - try setting the default charset to UTF-8:

var mail = new MailMessage();
mail.DefaultCharset = System.Text.Encoding.UTF8;