Update: I have created a free service to check your emails here:
Reduce time spent searching online for synonyms due to limited in-built functionality of Word.
Get the source: https://github.com/dunckr/word-thesaurus
Get API key: http://www.abbreviations.com/api.php
Tested on Win 7 and 8 on Word 2010.
For more check out: http://dunckr.com
Or on twitter: https://twitter.com/dunckr