This is a small task in my VEDBClient project. The email editor is a html editor, which has spell  check/thesaurus function like in MS Outlook or Open Office.
The html editor is an opensource project: http://69.10.233.10/KB/cpp/DHTMLEditor.aspx
The spellchecker is HunSpell, which is the default spell checker of OpenOffice.org and Mozilla Firefox 3 & Thunderbird.