How can I tell if a recipient has opened my email message?

When sending an e-mail, you can request a return receipt. This is a message that will let you know that your e-mail was delivered. You can either set this option on individual e-mail messages, or as a global option to apply to all e-mail sent.
Just keep in mind a 'Return Receipt' is a request and only works works if the person who receives your email clicks "OK" to acknowledge opening your. The problem is if they answer "No" to your return receipt request, you'll have no way of knowing if they've opened your email.


Return Receipt Settings in Thunderbird (Email Accounts Settings)

To access Thunderbird's email account(s) settings, go to Tools > Account Settings. In the Account Settings dialog, select the email account whose settings you'd like to modify.

Return Receipt Settings in ThunderbirdThunderbird's Return Receipts settings can be set for all your email accounts under the Compositions Options.

Thunderbird, however, lets you customize return receipts policy at the email account level. This means that whatever is defined under an email account's Return Receipts settings will supercede the global options set under Tools > Options > Composition.


Return Receipt Settings in Microsoft Outlook

You have two different options.
1. If you check the Request a delivery receipt for this message box, you will receive a confirmation when the e-mail is successfully delivered.
2. If you check the Request a read receipt for this message box, you will receive a confirmation when the e-mail is opened by the user. When finished, click Close.

To set this option globally, click on Tools, Options and then on the Preferences tab and the E-mail Options button. Now click the Tracking Options button. You should see the screen below. You have the same two options here. Set them to your preference and click OK.