From What is a “Sending Limit”?:
ProtonMail is not designed for bulk/spam emailing.
Keep your sending below (please note, these limits are suggestions. Your user reputation (as outlined above) is variable and will determine your unique sending limits):
Free Plan: 50 messages*/hour and 150 messages*/day
Plus Plan: 300 messages*/hour and 1000 messages*/day
Visionary Plan: User’s discretionProtonMail is currently not designed for high volume sending such as marketing blasts and transactional emails.
So even if you could use ProtonMail’s non-existent API, your “clients” would quickly break your send limit.
Email isn’t the only leaky pipe you need to patch if you are trying to create an anonymous Discourse experience. But you’ve saved some time, as that is not currently possible. 