Email-sending class for GAE
I'm not familiar with that framework, so I can only provide some general points:


You can rewrite the if statement to subject = ad.title or 'Reminder' (see here).
You are a bit inconsistent with spacings (before and after =, ,, (), etc.), and string formatting (your second and third logging.info)
I'd recomment spreading both those chained filters and overly long strings over multiple lines.
Maybe you could use some helper method to derive the HTML version of the message from the plain-text version or vice versa?