fix links to Mailgun docs (#9910)

mail.md

@@ -689,7 +689,7 @@ Some third-party email providers such as Mailgun and Postmark support message "t
         );
     }
 
-If your application is using the Mailgun driver, you may consult Mailgun's documentation for more information on [tags](https://documentation.mailgun.com/en/latest/user_manual.html#tagging-1) and [metadata](https://documentation.mailgun.com/en/latest/user_manual.html#attaching-data-to-messages). Likewise, the Postmark documentation may also be consulted for more information on their support for [tags](https://postmarkapp.com/blog/tags-support-for-smtp) and [metadata](https://postmarkapp.com/support/article/1125-custom-metadata-faq).
+If your application is using the Mailgun driver, you may consult Mailgun's documentation for more information on [tags](https://documentation.mailgun.com/docs/mailgun/user-manual/tracking-messages/#tagging) and [metadata](https://documentation.mailgun.com/docs/mailgun/user-manual/tracking-messages/#attaching-data-to-messages). Likewise, the Postmark documentation may also be consulted for more information on their support for [tags](https://postmarkapp.com/blog/tags-support-for-smtp) and [metadata](https://postmarkapp.com/support/article/1125-custom-metadata-faq).
 
 If your application is using Amazon SES to send emails, you should use the `metadata` method to attach [SES "tags"](https://docs.aws.amazon.com/ses/latest/APIReference/API_MessageTag.html) to the message.