Forminator Confirmation Mail only sending to some mail-addresses

Hi there

I set up a form with formulator for users to register to camps we will provide. Also created an automatic e-mail notification for the admin (me) and another one for the user, to the provided e-mail address from the form. I tested it out quite few times and with some e-mails it works, I get as a test user the notification e-mail with the confirmation. Fine. But as soon as I use my gmail-account, hotmail or outlook, it doesn’t work. There I get no notification e-mail.

What info do you need from me to be able to help me here? I’m not really familiar with WordPress, more like a beginner.

Thank you a lot
Cheers