If you can’t find your user because it’s not there (yet), you have to press on “Create new customer” and create it. Entering a mail address is not necessary.
Confusing is the error message. it should have been something like “You need to create User/Customer first” but not “invalid email of customer” in case of none existing customer since we create a ticket for a received call and we do not need email address.
Better way would be (maybe feature request) the User is created within ticket creation with minimal information as givenaname “new” and lastname “new2”. (This would make things speedup).
I think that’s a bad idea for users that hit enter to “search” a user.
This would cause Zammad to create dozens of duplicate (invalid) users, which is a nightmare in cleanup.