But would an attacker read a message marked as unread? A careful attacker would know that this alerts the owner of the mailbox, as he can't remember opening the

But would an attacker read a message marked as unread? A careful attacker would know that this alerts the owner of the mailbox, as he can't remember opening the message. His canary is a mail that has already been opened, so the attacker assumes he can look at it harmlessly.