I have sometimes started to respond to a message and forgot who the message was going to. And I said some things I shouldn't have. I click send and suddenly remember it is going to more people than I wanted. How can I "UNSEND".
Well sadly there is no "UNSEND" command. However if you use an email application like Outlook, Thunderbird, Mac Mail or others vs. using a browser you have options. These applications have various ways to at least give you some time to go into the applications and stop the actual send process.
Outlook as an example allows you to create "Rules". I have set up a rules that say for each email I tell it to send, it will store the email in the Outbox folder for 2 minutes. After 2 minutes the system will automatically send the email. You can control the number of minutes to pause before sending.
If you want the email to be sent without this delay I set up the rule to ignore the delay if I mark the message as important. It will then be sent when I click on the send button.
If I decide to stop the email from being sent within the time delay, I go to the Outbox folder and can delete, modify the email. Most of your other email applications have similar options that will provide your with the "UNSEND" feature you desire. Do a search for your email application and UNSEND or delay send, or some other text string that will accomplish your indecision.
Outlook - https://blogs.msdn.microsoft.com/edkatibah/2012/12/17/setting-up-a-delayed-sending-rule-for-email-in-microsoft-outlook/
GMAIL - https://productforums.google.com/forum/#!topic/gmail/ufA1WZgdSlo