You are adding the Exceptions to the Recipient URL Protection in the Click-time URL Protection Settings and getting error:
Maximum number of emails exceeded. Only 1500 emails are allowed. If you want this limit increased, contact your support representative.
Email Security.cloud
1500 email entries are allowed in the Exceptions list for Click-time URL Protection service.
Reduce the number of entries that is not greater than 1500.
If you want this limit increased, contact technical support.