Invitation email from App Store Connect not sent

I have recently tried to add a user to my developer account via the user page of app store connect but they have not received the invitation email whatsoever. I have also tried adding this same person to a test flight beta test but the email was not received then either. I have checked that he can receive emails and that app store connect is sending emails (by sending beta tests to other people). In addition, I have tried different browsers and computers but app store connect seems not to want to send any invitations to this particular email address. Any idea what could be going wrong?