Earlier quoted context omitted.
Isn't that the purpose of DKIM and SPF already?
ELI5: https://www.reddit.com/r/sysadmin/comments/16gvtdj/comment/k...
edit: looks like I had an extension that was redirecting to old.reddit.com, and it was old reddit that required login. Though when I turned that extension off, I got a "blocked by reddit security" error. ugggh.