Formidable Pro form builder - function to block email from certain domains

functions.php

<?php
/**
 * Block persistent marketers
 *
 * @param $errors
 * @param $posted_field
 * @param $posted_value
 *
 * @return mixed
 */
function iweb_block_persistent_marketers( $errors, $posted_field, $posted_value ) {
	if ( 85 == $posted_field->id ) {
		$blocked = array( 'example.com' );

		if ( filter_var( $posted_value, FILTER_VALIDATE_EMAIL ) ) {
			$domain = array_pop( explode( '@', $posted_value ) );

			if ( in_array( $domain, $blocked ) ) {
				$errors['field' . $posted_field->id] = 'Blocked email domain.';
			}
		}
	}

	return $errors;
}
add_filter( 'frm_validate_field_entry', 'iweb_block_persistent_marketers', 10, 3 );

You may use this plugin https://wordpress.org/plugins/block-email-formidable-form/