What does the log say?
In GAE, the sender of emails must be either a registered admin in the appengine console or a logged in google user. So if none of these conditions are true your email will not be sent.
Here's how I do it and it works for me (after having registered the email address as an admin in the appengine admin console):
from google.appengine.api import mail
...
message = mail.EmailMessage(sender='Kool Business <info@koolbusiness.com>', subject=article.title)
message.body = """
Hello!<br>Now your ad <a href="http://www.koolbusiness.com/vi/%d.html">%s</a> has been out for a week. If you already have sold your product you can remove your ad.<br><br>Some advice to promote your ad:<br>Change + Renew the ad and it will be on top of the list. You can also change text and price.<br>Change the ad if you only want to lower the price or change the ad text<br>Renew so that the ad will be on top if the list. <br><br>Best regards,<br>Koolbusiness.com
""" % (article.key().id(),article.title)
message.html = """
<html><head></head><body>
Hello!<br>Now your article <a href="http://www.koolbusiness.com/vi/%d.html">%s</a> has been out for a week. If you already have sold your product you can remove your ad.<br><br>Some advice to promote your article:<br>Change + Renew the article and it will be on top of the list. You can also change text and price.<br>Change the article if you only want to lower the price or change the ad text<br>Renew so that the ad will be on top if the list. <br><br>Best regards,<br>Koolbusiness.com
</body></html>
""" % (article.key().id(),ad.title)
message.to=article.email
message.send()