I'm A Blocked Sender On My Own Computer

Today I responded to an email from a friend and noticed that the sent message was marked (more or less) "This message is from a blocked sender." After a bit of poking I found that the blocked sender was me and that all of my email addresses going back several previous employers as well as my phone numbers were all on the block list in Mail. I'm at a loss for how this could have happened, I assume it was recently, but there are no time stamps on the entries in the blocked list. The only clue I have is that I got a spam message from Amazon this morning & when I tried to add the Amazon address to the block list it was already on the list (but despite having enabled blocked mail filtering and set it to move blocked mail to trash the message was in my inbox. Maybe as I poked around to try and make sense of that I might have blocked myself by accident, but my old email addresses aren't on my Contacts card any more. I'm most puzzled & would love to hear an explanation or even some theories.