When automatic emails are sent to several users at the same time, as in the case of notifications, they are sent to all recipients as blind carbon copies (field "BCC"), to guarantee the confidentiality of each recipient's email address. This is why, by default, automatic emails do not have a main recipient ("To" field).
However, this can pose a problem if your host, for anti-spam reasons, forbids you to send emails with this field empty. If this is the case, you can add a main recipient, who should be a valid address if possible (ideally your own).
If your host doesn't allow the use of the "Bcc" field, you can disable it. The "To" field will be used instead.