I discovered this and it has solved my problem.
If you've enabled the 'Always use https' setting in Gmail, you'll need
to install a patch for the Notifier to work with this setting:
Download the patch (.zip) <
http://www.google.com/mail/help/downloads/notifier_https.zip
>.
Open the folder.
Double-click the notifier_https.reg file.
Click yes when you're asked to confirm if you want to add the
information to the registry.
Restart the Notifier.
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