Looking at lifecycle hooks, I have found what I wanted. Here I can send a transactional email when a user register to the newsletter, using the sendinblue api (which makes more sense to me).
I share the code, if it might help someone else…
Before realizing that you found the solution for your problem, I was about to suggest to use mailgun - I used it a lot in many different contexts and was always very happy that I did
We had a lot of problems setting up Mailgun for a small application, it was even more problematic than SES. We are trying to make it work with SendGrid but as we have no a big company behind, it is hard to get an account there.