It would be beneficial if there was a way to flag email addresses in Koha as possibly invalid. That way, if a library receives a bounce-back for a given email, Koha could flag that email address as questionable. At that point, Koha could show an alert when checking out to that patron so the librarian can confirm the address is correct with the patron. An API endpoint that would accept an email address, find any instance of that email address in patron records, and flag it as potentially invalid would be very useful to integrate automation of converting bounce-back emails into warning flags in Koha.