Validate trough query or trough acymailing users

Hi there anyone knows if and how is possible to validate a field checking trough a query if the data inserted into a specific field exist into a db table or not?
In example into the email field: a user can register to an event only if his email is already written inside a specific table of the db, so we can restrict the subscription only to certain member group or (that would be better) acymailing users, so we can restrict the subscription only to the people that received an invitation.

Hi Step

For this, you will need to write your own custom validation rule, see eventbookingdoc.joomservices.com/basic-s...eld-data-server-side

Regards,

Tuan