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

um_email_notification_unverify_user.php

<?php

if ( ! function_exists( 'um_email_locate_template' ) ) {
	/**
	 * Locate a template and return the path for inclusion.
	 */
	function um_email_locate_template( $template_name ) {
		$blog_id = is_multisite() ? '/' . get_current_blog_id() : '';

		$template = locate_template(
			array(
				trailingslashit( 'ultimate-member/email' . $blog_id ) . $template_name . '.php',
				trailingslashit( 'ultimate-member/email' ) . $template_name . '.php',
			)
		);

		if ( ! $template ) {
			$template = wp_normalize_path( STYLESHEETPATH . '/ultimate-member/email/' . $template_name . '.php' );
		}

		return apply_filters( 'um_locate_email_template', $template, $template_name );
	}
}

/**
 * Add custom email template "Verified Users - Account is not verified E-mail".
 *
 * @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 E-mail', '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 ( ! array_key_exists( $slug . '_on', UM()->options()->options ) ) {
			UM()->options()->options[ $slug . '_on' ]  = empty( $custom_email['default_active'] ) ? 0 : 1;
			UM()->options()->options[ $slug . '_sub' ] = $custom_email['subject'];
		}

		// Template file.
		$located = um_email_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/

Method UM()->mail()->locate_template() has been marked private. Use custom function um_email_locate_template instead.