If you’re using a mail tag for a dynamic text or hidden input in the To: field, you will get warnings from Contact Form 7
If you click the link provided by CF7 beside the error notice, it explains why: Invalid mailbox syntax is used
In short, CF7 expects you to enter either a valid mail address, or a mail tag for an *email* form tag.
The CF7 DTX form tags are not email tags. So CF7 considers these to be “invalid”.
CF7 does not provide a way to register valid tags, so there is no way around the warning. But it’s just a warning, nothing is actually broken. As long as you are sure the tag you’ve created will always return a valid email address, you can safely ignore the warning.