The problem is coming from
if ($receiver->authorise('core.manage', 'com_message'))

This restricts the receivers to those with admin access and com_message access.

That's fine if it is the intended behaviour but the implication of the documentation is that it is not the intended behaviour https://docs.joomla.org/Publishing_Workflow/Scenarios/en#Setting_up_Notifications

It would probably be better to send the mail using getMailer instead of com_messages and thats a very easy change to make

I am working on a PR for exactly that. More details will follow later

ok I will undo what I have been working on

For reference I was following the pattern used for actionlogs as that would make it indeoendent of com_messages and allow for com_,mail_templates support

@brianteeman I like the idea of integrating it into the mail templates. Just I was wondering if we should keep the com_messages there as well. If we don´t use com_messages, what is the use of com_messages but that may be a complete different discussion ?

If you use com_messages for this then you are restricted to users who have admin access.

com_messages is still for used for many things