_MailItem.RTFBody Property
Office 2010
Returns or sets a byte array that represents the body of the Microsoft Outlook item in Rich Text Format. Read/write.
Assembly: Microsoft.Office.Interop.Outlook (in Microsoft.Office.Interop.Outlook.dll)
You can use the System.Text.Encoding.AsciiEncoding.GetString() method in C# or Visual Basic to convert an array of bytes to a string.