> addressed to an address that isn't mine.

This is because John was the envelope recipient: the one named in the SMTP protocol "RCPT to:" command.

There are ways that a perfectly legitimate e-mail you receive night not mention any of your addresses in To: or Cc:.

E.g. you are subscribed to a mailing list and some new posting (not part of some discussion you are in) arrives. It might just be addressed as "To: whatever-mailing-list@example.com". You received it because the mailing list robot used your e-mail as the RCPT to: envelope address at the SMTP protocol level.