Where is the history feature in Office 365? Can I see if recipients have read my e-mail to them?
A read receipt confirms if your email was opened by the recipient.
To request a read receipt for an outgoing message in OWA, click the extended menu while composing a message and click show message options.
Once the message options box appears, check Request a read receipt and click OK.
Please note that read receipts may be denied answer by recipients and therefore this feature is not a fool-proof way to ensure that messages have been received.