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In order to bypass the transactional email sending limits of the hosting plans, a third party transactional email service must be used.
To make that easy and automatically configured for all new hosted sites there could be a direct integration with such a service in the Hub. There’s a bunch of good ones like Sendgrid, Mailgun, Postmark, etc.
We as Agency account holders would sign up for an account with the email service and pay for it, and then set our own email sending limit per hosting plan. Users that exceed the limit could be offered the option to pay extra for additional email sends.
Each hosted site would automatically be configured to use the email service to send its emails.