You can cancel a mailing within 1 Hour of clicking "Start Mail."
After that, the mail will be printed and dropped in the mail. Once that happens, just like dropping mail into the post office mailbox, you can't reach back in and get it out!
If you start sending mail by mistake, you should see an undo button to stop it immediately. If you don't, open the lead and click pause mail.
Now if it has already been more than an hour... it’s already sent.