Debugging email for WordPress. Logs mail erorrs,

wp-mail-log.php

<?php
/**
 * WordPress Mail Debugger
 *
 * Log mail error message to debug-mail.log in wp-contents folder.
 * Log mail error to debug.log in wp-contents folder.
 */
function wp_mail_error_log( $wp_error ) {

	// Log mail error message to debug-mail.log file in wp-contents folder
	$fn = WP_CONTENT_DIR . DIRECTORY_SEPARATOR . 'debug-mail.log';
	$fp = fopen( $fn, 'a' );
	fputs( $fp, date( '[Y-m-d H:i:s] ' ) . 'Mailer error: ' . $wp_error->get_error_message() . "\n" );
	fclose( $fp );

	// Log mail error in the debug.log  in wp-contents folder
	error_log( '##--WP MAIL ERRROR --##' );
	error_log( print_r( $wp_error, true ) );
}
add_action( 'wp_mail_failed', 'wp_mail_error_log', 10, 1 );

x-wp-mail-log-force-mail-fail.php

<?php
/**
 * Call wp_mail function with invalid arguments.
 *
 * Can be used for testing
 */
function force_mail_fail() {

	wp_mail( '', 'Subject', '' ); // returns false, the email was not sent
}
add_action( 'init', 'force_mail_fail' );