Comment by geocrasher

Ok, so using a fake new gmail address the spammer used an instance hosted at mx5.messagingengine.com (probably a hacked or vulnerable mail server hosted at https://expedient.com/, cloud provider) to send mail via SMTP/gmail to a

   <list of several hundred random domestic email addresses - aol, hotmail, yahoo, icloud, gmail, etc.> 

There's nothing to indicate you were specifically targeted, but rather that they guessed your email address. It didn't bounce, so they kept sending to it.