"It wasn't broken until you fixed it": When SPF is so wrong, it's right
Modern (or maybe I mean young!) DNS admins don’t understand SPF as well as you might think. Another reason to not give them DNS tasks unless you’ve really proven the need. →
Your Marketo SPF entry doesn't matter (unless you're paying extra)
A pervasive Marketo myth is that you must have SPF and DKIM set up for your From: domains. Truth is you only need DKIM and never need SPF for your From: domain. (If — and only if — you have a custom envelope domain, you‘ll want SPF for that.) →