Even if you don't hit Marketo's actual daily limit, you'll likely slow your instance to a crawl. At my last company, we were trying to use a webhook to get email activity into a Redshift database using some Heap Analytics webhooks. Every time we sent a big email, the webhooks would take up all of the available processing power in our Marketo instance and it would delay other critical actions such as processing form fills, webinar registrations, etc. Don't do it.
There is no daily limit other than what the instance will bear under concurrent load.
So triggering on every send isn't the problem exactly -- it's that daily sends are not *themselves* metered. So every send on a slow day is fine, on a high-volume day it could be catastrophic. You can also set your webhook service to disable the trigger (using the Webhook is Called trigger to in turn call the REST API) when you go over a certain # of daily calls.
Thanks! Good to know. I'm working on a product similar to Heap's webhook for our customers (hence why I'm not very familiar with Marketo). It's good information to hear that this approach is not recommended for our customers. We probably just will not support Marketo activities in the near future.
From this integration (and another one I've worked on), Marketo really needs to improve their infrastructure. I'm not sure who to tell this to.