This is a (easily fixed) bug with 2.0.3
Basically you need to edit the file Products/CMFPlone/PloneTools.py (on windows usually C:Program FilesPlone 2DataProductsCMFPlonePloneTools.py). Open this in a text editor (you’ll need one which open UNIX style files – notepad will be all over the place), then change line 89 to
if not self.validateEmailAddresses(mto):
then you will need to save the file and restart plone.