If you send an email to a non-existant email address, you would get a Mailor Daemon message from the server.

If you send a vaild and correct email address to the correct person, but they have used up their server quota, the email will not be deivered and you would get a response from the server.

If you sent an email with a vaild email address, but not the correct email address for that person, then that email would get delivered to that email address even though it is not for the correct person and you would not get any notification because it was delivered as instructed.

So, if the email is not delivered then you would get a response email from the server.

There are a variety of reasons for non-delivery: http://en.wikipedia.org/wiki/Bounce_message

There is no method of any server knowing or indeed anyone knowing whether you have typed the correct email address for the recipient. That responsibility is down to you. Servers can only know whether it can or cannot be delivered for various reasons.

You can add a 'Return Receipt' to a message, but this does not guarantee a response as it is down to the recipient as to whether they choose to acknowledge the request. This would also only tell you that the message was delivered. You would only know it went to wrong person if the recipient responded back telling you.

See more info:

• http://kb.mozillazine.org/Figuring_out_whether_the_recipient_read_your_message