Is there any alternative for it that works with Plone 6 with Volto, what would be an alternative?
i also need this man, did you happen to find an alternative ?
An alternative you can explore is GitHub - collective/collective.listmonk: Listmonk newsletter integration for Plone . It is a basic integration for the opensource Listmonk newsletter application. It runs stand alone. And the add’on communicates over its api for (un)-subscribe, and sending.
kitconcept developed this for a client. I haven’t installed and configured it myself in another project, I don’t know how much of the newsletter composition/gathering from content is also implemented in the collective package or was part of the customisations for the project.
Products.EasyNewsletter is a great add’on, but is mature, uses and is rooted in a lot of ClassicUI and also manages the sending. This is something I wouldn’t recommend doing from the Zope proecess itself [edit: nowadays]. Listmonk and other specialised newsletter apps have much better send-controls, for example to avoid overloading mailservers and spam positives.
If I construct a newsletter in Plone itself, how is it to use 'some kind of service' just for sending?
In other words: I have made a (working) view that constructs and sends email / newsletter, so the goal is to make sure as many as possible gets it.
PS: The mailing list / add / remove users would be taken care of by my client ( users /groups in Plone or similar)
We have it running in a Plone 6 ClassicUI. You could configure an access to it and use it there in the classic interface. That said I am not sure how well rendering of the Blocks based Volto Content works.
I'm currently looking to implement brevo. You can either send a newsletter with a template or use your own HTML. My tests worked great and the price seems to be reasonable. API is easy too. Hosted in France and fully GDPR compliant.
Do you have any idea about how a 'local' application 'work' regarding 'spam positives etc'?
I'm sorry, but I don't understand what your question is here. 
. Could you please rewrite it in full sentences, one per question?