It is not Thunderbird who notifies you - it is the server because it is the server which tries to deliver your emails. If the email is sent to a valid email address, but it was not the correct email address for the person you intended then it has been delivered and you will not know about because the recipient was the wrong person and they may just delete email. If the email was sent to a valid and correct email address, but could not be delivered because the recipient Inbox had used up server quota, then you should get a message from server stating it could not be delivered. If email address does not exist - it cannot be delivered and you should get message saying it could not be delivered. If email address was valid and sent to correct person, but the server spam filters intercepted it, then they would just delete the email and you would not know what happened. If email address was valid and sent to correct person and they recieved it, you would not know until they responded. There are a couple of setting which you could try to use: In Write message - Options > Delivery Status Notice If server allows - it may respond with a notification that the message was delivered to recipient server, but it does not tell you whether the receiving server thought it was spam and killed it or whether recipient actually received it in either their Inbox or Spam folders. In Write message - Options > Return Receipt This sends a request for a Return Receipt with email. Some servers do not permit users to send 'Return Receipts' as it counts towards email send limits, so they auto disable it and recipient is non the wiser. Even if recipient does see the request, they are not obliged to act upon the request.