This code adds custom email template "Verified Users - Account is not verified" to be used in the "Ultimate Member - Verified Users" extension.

um_email_notification_unverify_user.php

<?php

/**
 * Add custom email template "Verified Users - Account is not verified".
 *
 * @param  array $emails Emails list.
 * @return array
 */
function um_email_notification_unverify_user( $emails ) {

	// New email templates.
	$custom_emails = array(
		'unverify_user' => array(
			'key'            => 'unverify_user',
			'title'          => __( 'Verified Users - Account is not verified', 'um-verified' ),
			'description'    => __( 'Send a notification to user when his account is not verified', 'um-verified' ),
			'recipient'      => 'user',
			'default_active' => true,
			'subject'        => 'Your account is not verified on {site_name}',
			'body'           => 'We have reviewed your verification request. '
			. 'Unfortunately we can not verify your account now. '
			. 'Please update your profile and fill all required fields.<br /><br />'
			. 'To view your profile click here: {user_profile_link}<br /><br />'
			. 'Thank You!<br />'
			. '{site_name}',
		),
	);

	foreach ( $custom_emails as $slug => $custom_email ) {

		// Default settings.
		if ( '' === UM()->options()->get( $slug . '_on' ) ) {
			$email_on = empty( $custom_email['default_active'] ) ? 0 : 1;
			UM()->options()->update( $slug . '_on', $email_on );
		}
		if ( '' === UM()->options()->get( $slug . '_sub' ) ) {
			$email_sub = empty( $custom_email['subject'] ) ? $slug : $custom_email['subject'];
			UM()->options()->update( $slug . '_sub', $email_sub );
		}

		// Template file.
		$located = UM()->mail()->locate_template( $slug );
		if ( ! file_exists( $located ) ) {
			wp_mkdir_p( dirname( $located ) );
			file_put_contents( $located, $custom_email['body'] );
		}

		$emails[ $slug ] = $custom_email;
	}

	return $emails;
}
add_filter( 'um_email_notifications', 'um_email_notification_unverify_user' );


/**
 * Send a notification to user when his account is not verified.
 *
 * @param int $user_id User ID.
 */
function um_email_notification_unverify_user_send( $user_id ) {
	$userdata = get_userdata( $user_id );
	$email    = $userdata->user_email;
	um_fetch_user( $user_id );
	UM()->mail()->send( $email, 'unverify_user' );
}
add_action( 'um_after_user_is_unverified', 'um_email_notification_unverify_user_send', 20, 1 );

This gist is a part of the article How to add and use custom email templates.

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/