You can choose to receive notifications when your emails are read by the recipient.

To turn on read receipts for all emails that you send, open the app settings (View) - Email management, and turn on "Read receipts".

If you want to turn read receipts on or off when sending an email, tap on the action menu (circle with 3 dots button) in the top right of the screen and choose the read receipt menu item. You may need to tap on the "..." button in the top left of the send email screen to enable more advanced button options.

You can check emails in your sent folder, to see when you the reader opened your email. Read receipts are saved in sent emails. Tap on the spectacles icon on a sent email (if showing) to see when the recipients read your email.

Note: You may not always get a read receipt notification when your email is read. If the email reader has turned off 'Load remote images' then you may not get a notification. Apple Mail may also block read receipts when the 'Hide IP address' setting is turned on.

You may also see multiple copies of the same email in your sent folder when you send to multiple email addresses. This is normal. Each recipient will only receive one copy of the email you send.
The reason for multiple emails to have a different read receipt id for each recipient.