Self-sending spam emails - mark as junk or just delete?

You should mark these as spam, or setup a rule so that if you ever do need to send anything to yourself you can use a keyword in the subject so it isn’t junked. Take a look at ‘rules’ in the Mail.app preferences. Hope this helps!

Also: no, it will not affect the sending of email in any way.

My concern, both for Apple Mail and for Gmail, is that by marking as spam, I am adding my own email address to the spam list. You are saying this is not the case?

I am more concerned that Apple Mail will start seeing my own messages as spam because of the email address, or that I would be adding my Gmail address to some global Gmail spam database. Not so?