When you send an email before you're ready to send.
You might hit send
accidentally or your cat walks on the keyboard and sends it for you or the next thing you know, the email has been sent even though you haven't even touched the keyboard.
The prematurely send-jaculated mail
definitely isn't ready to be sent as it might be halfway written, have a lot of typos, or be too rant-y or stupid to be sent, hence causing
embarrassment when the email reaches the recipient.