Enable logging of SMTP errors

send-smtp-email.php

@@ -34,3 +34,11 @@ if (defined('SMTP_HOST')) {
 	}
 }
 
+/**
+ * Handler to log email failures.
+ */
+function log_mail_failure($wp_error) {
+    error_log('SMTP error: ' . print_r($wp_error, true));
+}
+
+add_action('wp_mail_failed', 'log_mail_failure', 10, 1);