I want to know if an email I sent has been opened. How do I do this

Sorry to say, I'm afraid Mail has no such facility.

Short answer, you don't.

There's a pseudo-standard for read receipts that purports to tell you, but it's easily circumvented, either by the recipient using a mail client that doesn't support the 'standard' (hint: most don't), or by the recipient turning off read reciepts. In short, not getting a read reciept does not mean the message wasn't read, and therefore the whole concept is junk.

There are some other options that might get you further along... for example, if the message includes images or other HTML content then you could check the web server logs and track back who requested the image content. This does, of course, require that the user has their mail client set to display images and 'rich content' - if they don't download the image you can't know.

If the recipient is in your domain (or in a domain you control) then you could query the mail server logs - you'll see the message saved to the user's mailbox and can be pretty sure it's downloaded to their mail client the next time they POP or IMAP in. However this only implies it was delivered, not read, and is only valid for mail domains you control (you don't have this level of visibility into other mail servers).

At the end of the day, the only foolproof answer is: ask.

There are online services that you paste a image into your email and when it's accessed form the server you'll get a notification.

I don't know of any, but I do know they exist.

Serach for "email notification service" or something like that.

I do not know if this will help

<http://www.macobserver.com/tmo/article/imessages_enabling_read_receipts_on_macs_ ios_devices/>

But also as Camelot says, there is no way to force the receiver's mail client from refusing to provide the return receipt response.

if the person does not get them and read them it causes me big problems

I feel your pain, however, there is still a world of difference between 'get' and 'read'. People may well get (as in 'receive' the message, the might even open it (which would trigger a read receipt for any mail client that supported it), but actually reading it? That's a whole different question. No matter how much you try, people won't always read the message. Even acknowledging receipt doesn't mean they read it. 🙂

The best solution I know of to this is to include a test at the end of the message (or somewhere in the middle)... something bizarre such as 'today, the sky is purple' and have them tell you today's sky color - that forces them to read the email and if they can't tell you the color of the sky you know they haven't read it. 🙂