Two separate budgets: transactional email sends (API) and form submissions. A form submission never eats into your send budget, and vice-versa.
At the form limit
Submissions are never dropped. Once you hit the cap they're saved and held — your visitors still see a success page, and the leads are released the moment you upgrade. No lost leads, ever.
At the send limit
API transactional sends return a 429 and are rejected — no surprise charges. Resets on your next billing date.
At the mailbox limit
The Connect button is replaced by an Upgrade prompt. Existing mailboxes keep working normally — only adding new ones is blocked.