Thanks for your prompt response. I wasn't filtering for 429, but only for 503, so that might explain it. 
This is my current countermeasure against overloading the system:

If you follow all that, you should be good. If you still see throttling / ban, let us know. If you give me the User-Agent of your script and the time at which you received the throttling / ban response, and I can have a look into the logs.

Where do I let you know? Is this email list the right place to do so?