CF7 doesn’t send email

If I fill out the form and press submit, a mail should be sent to the address entered. Unfortunately, this only works with my gmail address, but not with any other address. (Since I don’t have another gmail address, I can’t test if it has anything to do with gmail.)
I have already tried all the possibilities. In CC, BCC, normal and in the additional header, multiple addresses simultaneously,… But the only mailbox that receives the mail is my private one.
My original thought was that the customer and I both get the same mail with the information. So like this: “To: [your-email]” and “Additional Header: Bcc: xxx@gmail.com”
Can someone please help me get this done?

The page I need help with: [log in to see the link]