Which extension in Mozilla Firefox will help you receive delivery notifications through the Gmail web interface?

Question

gmail

Dear users, please tell me an extension for Mozilla Firefox, with which you can receive notifications about the delivery of a letter to the recipient's mailbox through the Gmail web interface. I draw your attention to the fact that this is not about the fact of opening and (or) reading the letter, but about the delivery of the letter to the recipient's box. Perhaps, from a technical point of view, it is more correct to say "to the recipient's server". After all, this letter can be opened / read much later.
I know about the Right Inbox extension, but it only gives feedback after the recipient opens the email. Ideally, I'm looking for a free solution and something like MailTrack for Gmail available for Google Chrome.
Thank you, I will be glad to any answer!

Answer 1

No extensions needed. Any sane mail server sends a bounce letter if it could not deliver the letter to the recipient's server. Google is no exception. If after sending within a minute nothing came back, then everything is ok.