Hi, to test IF the provider is working correctly you need a test script, PHP, which invokes the mail function:
mail(‘your address’, ‘subject’, ‘example of test message’)
your provider can this script in your hosting space and test it. If the email is delivered and since WP uses internally this kind of function call, it means something in WP is not working. It could be and SMTP plugin installed, or a plugin which change the default WP behavior. But since before everything was working I suspect it’s a provider problem.
Are you able to create such script to test the default PHP mailing? Have you a friend able to do it?
You can even try a password recovery from your login page, just to see if that email is delivered.
Did you changed the sender address in Newsletter main settings? The sender address could be filtered by your provider, usually a good choice is to use an address in your domain.
Stefano.