Is there an equivalent of profile substitution method or Preview of Emails for Transactional Emails in ACS with live API payload data after Pausing the Email and releasing the Email after the validation is done?
Is your transactional email of type Real-Time or of type Profile. I suspect if it of type Profile. When you mentioned, live API payload, what do you mean by that? That's it is a true call from the 3rd party system acting as it should and at last minute you want to swap the email address to a test email address? Why not having a true test user created in the 3rd party system, such test user would be propagated to ACS in the agreed way and from there, you could use the test user instead.... Is that been explored? We've got customers using their own email address as true user of their 3rd system (ECommerce for instance)