There is no one correct format for that. Main messages can have different parts, each in its own "format": text, graphics, and more. One of the content types is HTML, but some use RTF (way too obsolete). After all processing, everything is represented and sent
as a plain text, but parts of the text can be encoded to some plain-text representation, typically, using base64. For example, HTML is already the plain text, so base64 is not used.
You are talking about something unrelated, the UI. As far as mail is concerned, it's a matter of convenience. If you want the user to put all content types in a plain-text format and encode all the content types the way user wants, it will, in principle, perfectly work, but the user would not want to do all that. If you use, say,
RichTextBox, it will help you to write user's content as RTF, but it would be too limited.
The best approach is the one provided by some base mail clients. It gives you some option for entering formatted and unformatted text and options for your main-part content type (usually HTML or plain text). It allows the user to add parts by exporting separate files and to specify
content disposition for each. Content disposition will help to choose, for example, if a PNG image is represented with "save as" or as a part of document flow inside the mail content. This way, the user can use any external editors (the same way as one would use an image editor) to edit separate parts.
Please see:
http://en.wikipedia.org/wiki/Content_type[
^],
http://en.wikipedia.org/wiki/MIME[
^],
http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html[
^] (pay attention for "content disposition"),
http://en.wikipedia.org/wiki/E-mail[
^].
—SA
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