Hello,
Towards the bottom of my message, you'll find another option, so read it through.
But, first, I'll address the Return Receipt feature you specifically mention.
Mail does not have a return receipt function. And, for the most part, it wouldn't do you any good if it did have one.
The reason is that it is up to the receiving e-mail program (more specifically the user) to return the receipt. So, unless they specifically have their program set to do it automatically, then they will be asked each time and given the opportunity to decline.
I have received several e-mails over the years that had return receipt attached to them.
Each time, my e-mail program asked me if I wanted to acknowledge that I had received them. In almost every case, I chose "No".
The only time I will click on the "Yes" button, is when I am in the middle of a business transaction with that person, and I feel it is important to let them know that I received the message.
Otherwise, I feel it is a bit of an invasion to expect this return receipt.
I know that if my e-mail program tells you that I have received the e-mail, that you will then expect an immediate reply.
But, I may not be prepared to immediately reply to your question. Perhaps I need some time to think about it.
But, because my e-mail program told you I opened the message, I know that you are now expecting an immediate reply. And, if I wait for several hours or a day to reply, then you will feel like I'm ignoring your message.
If you just don't know whether I've received your message yet, then you won't be sitting there anxiously awaiting my reply and feeling like I'm neglecting you.
So, with that in mind, I will always elect not to acknowledge receiving the message when it comes in (unless I feel it important to do so). And, I'll reply to the message when I feel that I have gotten my thoughts together enough to reply properly.
But, with all that said, there are two methods to verify that a message has been sent. Not received, but sent:
1) Activity Viewer
In Mail, go to the "Windows" menu, and click "Activity Viewer". You will now be able to see every send, receive, and other operation that is going on at the moment.
2) Blind Carbon Copy (BCC)
Start a new message, and then go to the "View" menu.
Now, select "Bcc Address Field"
This will give you the BCC address box in your messages from now on.
Anytime you place an address in the BCC field, the message will also be sent to that address. But, the receiver will not see that address in their copy. This is often used to send messages to multiple people without the headers revealing that other people received it (and keeps the address private).
So, place your own address in this box, and you'll get a copy of it (without the message revealing that you also sent it to yourself).
Now you'll know for sure that it did send because you'll receive a copy.
Now, this doesn't prove that they received it. Some server along the way could still lose it.
But, you will know for absolute certain that you sent it.
Hope this helps.