A Google search will reveal there are apps that enable you to track your emails, like bananatag, streak, and spypig.
However, I think the best way to find out if your messages have been received is to ask the intended recipients. Send your messages, and wait for a response. If none is forthcoming, send another short message saying you are just checking if the system is working properly and ask if they got your last message, quoting the subject line and the date you sent it.
If there's still no reply, don't waste time trying to provoke a response: keep them in the message loop and get on with building your business and finding new, motivated people who will respond to your messages.
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A Google search will reveal there are apps that enable you to track your emails, like bananatag, streak, and spypig.
However, I think the best way to find out if your messages have been received is to ask the intended recipients. Send your messages, and wait for a response. If none is forthcoming, send another short message saying you are just checking if the system is working properly and ask if they got your last message, quoting the subject line and the date you sent it.