Keep in mind that default Gmail allows webhooks for any changes (email received but also changing labels, etc), for free using Gmail pubsub. I use it a lot because it's the only way of getting programmatic notifications from credit card purchases (turn on purchase alerts to all cards, send to Gmail, have a filter archive but capture the reception in webhooks. Parse with simple regex)

Super fast low latency very satisfying. Pubsub scales well and free :)