The truth is you cannot edit an email once it has been sent.
Imagine your email as a letter you have put into a mailbox. Once the mail is picked up, you are unable to take back the letter. This same system exists in Constant Contact.
Some email clients, such as Outlook and Gmail, have the feature but there is a caveat. Outlook, for example, only works within the same Outlook Exchange system. If it is sent outside of that ecosystem, it won’t work. Gmail doesn’t actually retract the sent email, but (if you have the feature turned on) will delay sending the email up to 30 seconds for you to hit the undo button.