It's careless to send an email to the wrong person
Jacob is writing an email to a coworker complaining about their boss.
Jacob wants to vent to a coworker.
Jacob makes sure the email is addressed to the coworker before sending it.
Jacob doesn't check the email before sending it and accidentally sends it to the boss.
Jacob's coworker replies that he understands Jacob.
The boss reads the email and is annoyed with Jacob.