I'm not sure what the script is currently using for sending transactional emails. But from experience, I know that Postmark is one of the best providers for sending transactional emails. I've been using them for years, and they're really good AND very developer friendly.

You can check their documentation here: https://postmarkapp.com/developer

Ideally it would be nice to be able to use their API but if that's too much work, then simple SMTP would be good too. As long as we can use Postmark.

If you have already something else integrated, then we can use that - but if nothing is yet being used for sending transactional emails, then Postmark API is the best because you can create separate streams of emails (transactional, marketing, etc.).

Let me know if you have any questions or need me to elaborate on anything - happy to help.