We're in the process of cleaning up our SPF record (too many entries) and it let me down the rabbit hole that is SPF/DKIM. I felt like I understood it until I tried sending some test emails out of curiosity. Our SPF is currently on our main domain (and I think our branded domain as well). Out of curiosity, I sent my personal email an email from one of our competitors and it showed up and wasn't marked as spam! (though it did have a "via mktdns.com" after the sender email). Why am I able to do that without the message being marked as spam? Shouldn't gmail (my personal email) see that our Marketo instance isn't authorized to send from that domain and the email be marked as spam?