Maybe this ASP function to strip HTML code would work.  It shouldn't be hard to convert:

' Function Strip All HTML Tags
' Author Abd Shomad:
' http://www.4guysfromrolla.com/webtech/abdshomad.shtml
Function StripHTMLTag(ByVal sText)
    StripHTMLTag = ""
    fFound = False
    Do While InStr(sText, "<")
        fFound = True
        StripHTMLTag = StripHTMLTag & " " & Left(sText, InStr(sText, "<")-1)
        sText = MID(sText, InStr(sText, ">") + 1)
    Loop
    StripHTMLTag = StripHTMLTag & sText
    If Not fFound Then StripHTMLTag = sText
End Function

http://www.4guysfromrolla.com/webtech/100199-1.shtml

string plainbody = Editor1.PlainTextWithLinefeeds + endtagtxt;

string stripped = Regex.Replace(plainbody,@"<(.|\n)*?>",string.Empty);

msg.PlainMessage.Body = stripped;