maillog.php

<?php
/*
 * add any mail errors and error messages to DEBUG log
 */
add_action( 'wp_mail_failed', function ( $wp_error ) {
	/**  @var $wp_error \WP_Error */
	if ( defined( 'WP_DEBUG' ) && true == WP_DEBUG && is_wp_error( $wp_error ) ) {
		error_log( 'Email - wp_mail error msg : ' . $wp_error->get_error_message() );
		$a=$wp_error->get_error_codes();
		foreach ( $wp_error->get_error_codes() as $error_code ) {
			$b=$wp_error->get_all_error_data( $error_code );
			error_log( print_r( $wp_error->get_all_error_data( $error_code ), true ) );
		}
	}
},
	10,
	1
);
/*
 * Log all out going emails before sending
 */
add_action(
	'wp_mail', function ( $mail ) {
	if ( defined( 'WP_DEBUG' ) && true == WP_DEBUG ) {
		error_log( 'Email - wp_mail about the send email : ' . print_r( $mail, true ) );
	}
},
	10,
	1
);