This method will introduce the Recall This Message feature to recall a sent email before your recipients reading it, and then resent the sent email with you updates in Outlook.

1. In the Mail view, click to open the Sent Items folder, and then double click to open the sent email you need to update.

2. Now the sent email is opening in the Message window. Please click Message > Actions > Recall This Message. See screenshot:

3. In the opening Recall This Message dialog box, please check the Delete unread copies and replace with a new message option, and click the OK button. See screenshot below:

Note: It’s optional to check the Tell me if recall succeeds or fails for each recipient option.

4. Now the sent email is opening in the editable mode. Please update the sent email as you need, and then sent it.

5. Close the Message window of the specified sent email.

Notes:
(1) This method can update a sent email only if your recipients have not read it.
(2) If you have checked the Tell me if recall succeeds or fails for each recipient option in the Recall This Message dialog box, you will receive notification emails as below screenshot shown:

Recall or retract a sent message in Outlook

Delete/recall a sent email before recipients reading in Outlook