E-mail Confirmation

Is it possible to determine if an email was really sent?

The issue is, the user click in “send email” button but after this moment he notice that the Internet is unavailable.

The email is sent if you don’t get an error message. When the internet is unavailable or becomes so you will get an error.

On IMAP and Exchange systems you could check if the Mail was delivered to the Server, by looking into the apropriate boxes. In all other cases, only the Mail Server knows if a Mail has really been sent.