you only read enough to identify that this was no work related - honest
I believe the correct response is to return the email to the sender, indicating that you had only read
enough to identify that this was not work related.
Then remind the sender that personal messages should be sent using personal email.
Nobody will believe that you didn't read the whole email, but the message that you don't
appreciate having personal stuff like this get misdirected to you will have been delivered.