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.
Co-worker to another: I just hit the wrong
key and sent half an
email to the boss that I shouldn't have!
Another: Premature send-jaculation is dangerous, hope you
don't lose your job dude