I have been getting constant spam from events@eventful.com. I have marked that address as a blacklisted sender, but I continue to get messages from it. Why is this?

getting this on your chess.com email address?

Yes.

There are certain types of email that are sent across different email addresses (spammers can mask the true address of where the email originated from). The chess.com server might register the email as coming from the fake address. There is software to find out the real address emails come from but I'm not sure if that will support webmail (e.g Gmail, chess.com email, etc.)

Yes, spoofing. Still, if I block the "fake address", if chess.com deems the email as from that address, then the message will be marked as spam.

I like a healthy dose of transcritical bifurcation in my morning coffee.

That has nothing to do with spamming.

Ok...err...let me clarify what I said before. The email interface displays the fake address but maybe the server identifies it has the real address. I have never received such spoofed emails so I won't really be able to help here.