After much struggle with this, I finally decided to continue my project with one of those SMTP services. I still don’t know what was wrong with this, regardless of the package being so old and unmaintained (because it worked in local, not when deployed to Netlify).
Could this have been a issue and incompatibility with the port used to send the mail?
We don’t maintain that package, or have any expertise in it. The documentation of that package doesn’t say anything about requirements on give any useful details. If you find any such info, let us know and we can check accordingly.