Ultimate Member - Only use a Selected email domain for registration

um-email-registration.php

<?php
/**
* Ultimate member - Restrict registration based on email domain.
* URL: https://wordpress.org/support/topic/email-domain-restriction-at-registration/
*/
add_action( 'um_submit_form_errors_hook__registration', 'um_register_email_provider_restrict', 99 );

function um_register_email_provider_restrict( $args ) {
	// List the email providers you want to allow
	$allowed_domains = array('@gmail.com', '@hey.com');

	// For each domain provider check if the domain provider is allowed for registration.
	foreach( $allowed_domains as $domain ) {
		if ( isset( $args['user_email'] ) ) {
			if ( !strstr( $args['user_email'], $domain ) ) {
				$message = sprintf( __( 'Only use the email domain %1$s, %2$s for registration', 'ultimate-member' ), $allowed_domains[0], $allowed_domains[1] );
				UM()->form()->add_error( 'user_email', $message );
			}
		}
	}
}