You want to know if your e-mails (maybe professional ones) have reached safe destination. You can configure Thunderbird to request a return receipt for your sent emails, here's how to proceed:
For all your mail accounts
- Open Thundebird.
- Click on Tools.
- Options > General Tab.
- Next to the "Determine how Thunderbird handles return receipt", click on the "Return Receipts" button.
- Check the box next to "When sending message, always request a return receipt".
For a specific mail account
In the case that your are managing multiple mail accounts in Thunderbird and want to enable this setting for a particular account.
- Click on Tools > Account Settings
- Select the account of your choice and go to the "Return receipt".
- In the right-pane, select "Customise return receipts for this account" and check "When sending messages, always request a return receipt".