Why do registrants see "A valid email address is required." even though they have entered a valid email address?

If a would-be registrant is shown the error message "A valid email address is required." after clicking the register button even though they have entered a valid email address in the Contact Information section, it may be due to an issue with the Minimum Tickets and Maximum Tickets parameters for a specific ticket. For further details on the problem and the solution, see Why are there no quantity options for a ticket in the drop down of the Qty column for a registration form?

If that does not resolve the issue, please submit a ticket and we'll be happy to assist!