Most email clients can’t normally undo an email sent by mistake and this same rule follows for messages sent through Luminate Online.
When sending emails, it's important to not submit a delivery until you’re absolutely sure you want to send the email. Returning the message to a Draft state and making edits will not have any affect on what has already been sent out. This the same for Message Viewer links in already delivered emails, as these are generated at the time of delivery.
If you do send out a message and later wish to edit the content, we recommend first copying the message. This will give it a new Message ID, allowing you to send to all constituents that received the original message. You can then make edits as needed and send the new message to the same recipient list as the original. To avoid having your message appear as spam, it's recommended to add wording to the subject or message content that let's the recipient know this message is a resend/correction.
To help prevent incomplete or incorrect message from going out, you may consider adding a Reviewer List for your emails.