Notes, Notings, and Common Refrains

I second this. Spammers use their ill-gotten email lists not only to spam to, but to populate their forged from: fields. So when a spam like that bounces, it goes back to the spoofed "sender," in this case yourself.

There is also spam that simply makes itself look like something you ostensibly sent which failed and came back, for the reason that you're more likely to actually look at it if it seems like the result of a mistake on your part. In an industry that sends out billions of dodgy emails per second, any trick can raise the percentile of their ill-gotten returns.

Either way, it's harmless to you.