of course I know it's not illegal to ask and get a name which is used by plugins external to the WordPress directory and I don't want to suspect that the other Newsletter Pro plugin did it intentionally (even if naming a plugin like another one already existent from years sounds not so nice).
By the way, I'll patch Newsletter Pro so it cannot be updated by WordPress. As far as I know there are not best practices on how a plugin owner should do it. Probably it would be interesting for the WordPress team (which any of us should never end to thank) to add something that enable a plugin to stop the new version notices.
The problem of conflicting names cannot be solved. If I rename my Newsletter Pro as Newsletter Pro X, someone could always register a conflicting name. Changing the name, even if possible, it's not the solution (for me at least). In my case changing the folder name of Newsletter Pro will be extremely complex since it's not a two page plugin (ok, developer, I should used more defines... I agree).
Actually the other plugin is suspended, but I'm pretty sure it will be re-enabled. This is a lesson and a problem to any developer creating plugins not published on WordPress repository (even if not commercial) and not only plugins...!
Version 3.0 will solve definitively that problem being a single plugin with modules not subjected to update notices... I suddenly start to rewrite them this way!