Message to <3@2031342> was automatically rejected: spam

I received this bounce-back from the Mail Delivery Subsystem:

Your message to <3@2031342> was automatically rejected: spam

Who is  <3@2031342>? Certainly not someone i communicate with. I've not seen that format for addresses before.

And the message was bcc'd to 2 of my students and cc'd to my co-prof. Why was it screened as spam?