I do not think this is mere spoofing, and I do not think it's exactly like Receiving spam from myself
Here is a diff of a test email I sent to myself(on the left), and the spam email(on the right).
Note that the green boxes are different between the two. I blocked it out because I'm not sure if there's a security issue in revealing it.
The thing that makes me think it was my mac, is that the "Recieved:" headers are exactly the same with the exact same IP address.
another note: The primary purpose of this mac for me is to run a windows instance via parallels.
Any idea what this is? Malwarebytes for mac didn't find any malware.
Note: I have 2-factor authentication. I don't think it's a simple matter of guessing my password.