The following code requires users to register with a business email only and blocks popular email services, such as @gmail.com, @yahoo.com. ect. The code below requires a user email to be collected during registration.

Block the email address by domain.php

<?php

/**
 * This example shows how to validate the email address by domain on registration.
 * The code below requires a user email to be collected during registration.
 *
 * Change the array $forbidden_email_domains - add or remove domains you need.
 * Add this code snippet to the end of the file functions.php in the active theme directory.
 *
 * Ultimate Member documentation: https://docs.ultimatemember.com/
 * Ultimate Member support (for customers): https://ultimatemember.com/support/ticket/
 */

/**
 * Custom validation for the field "E-mail Address" by domain on registration.
 * @author  Ultimate Member support <support@ultimatemember.com>
 * @param   array  $args
 */
function um_validate_email_domain( $args ) {

	// Add forbidden email domains here
	$forbidden_email_domains = apply_filters( 'um_forbidden_email_domains', array(
			'gmail.com',
			'yahoo.com',
			'hotmail.com'
			) );

	// Change error message here
	$message = __( 'You can not use this email domain for registration. Please use a business email address.', 'ultimate-member' );

	if ( isset( $args['user_email'] ) && is_email( $args['user_email'] ) ) {
		$email_domain = array_pop( explode( '@', trim( $args['user_email'] ) ) );
		if ( in_array( $email_domain, $forbidden_email_domains ) ) {
			UM()->form()->add_error( 'user_email', $message );
		}
	}
}
add_action( 'um_submit_form_errors_hook__registration', 'um_validate_email_domain', 20 );