I would try one of the following:
1) Use the WinForms WebBrowser control in edit mode
Following VB.NET code is taken from Stackoverflow question: winforms html editor.
Me.WebBrowser1.Document.OpenNew(False).Write("<html><body><div id=""editable"">Edit this text</div></body></html>")
'turns off document body editing
For Each el As HtmlElement In Me.WebBrowser1.Document.All
'turns on editable div editing
.SetAttribute("width", Me.Width & "px")
'turns on edit mode
Me.WebBrowser1.ActiveXInstance.Document.DesignMode = "On"
'stops right click->Browse View
Me.WebBrowser1.IsWebBrowserContextMenuEnabled = False
Note that this control works only when Internet Explorer is installed. It also has some shortcomings, but it's free and it may be enough for basic editing. There is also a free (Ms-PL license) wrapper around the WebBrowser control at codeplex. I would use this when the end user's environment can be controlled (you need the IE installed) and in case that I need only a basic editing.
2) Try some supported commercial component
XStandard supports WinForms and has quite a good reputation. (I've not tested it personally). I would use it if I have to distribute the app to many users and/or when XHTML compliance is needed.
3) Use your mail client and save them to disk in EML format
Use your favorite mail client for composing HTML messages. Save the message to the server. Download message via the IMAP and save them to the dist as EML file. EML files are human readable (sort of) and can be used as a template. I would use it if I have to prepare only few emails and or if modification of the produced HTML (e.g. adding mass mailing merge fields) is required.
Tip: you can use IMAP Browser sample to get a message content in EML format. The
Raw message tab shows the message in the EML format.