I think that you have an overestimated opinion about the vendors' SDK, not all of them have acceptable quality, but let's leave it to the discretion of the deve

I think that you have an overestimated opinion about the vendors' SDK, not all of them have acceptable quality, but let's leave it to the discretion of the developers themselves. What is really important for our clients who use the EmailJS service from the back-end is flexibility and freedom of choice. If vendor "X" suddenly stops working for you or if there are any problems, the migration to another vendor will take several days. With the EmailJS service, about 10 minutes: add another service and move your traffic to it. Indeed, someone could copy your code, but they will only be able to send your templates, with your content, and will not be able to send a custom email with their own content (spam). Which is absolutely not interesting for attackers.