Recently, our Marketo emails are showing 'Caution: External Email' at the top of emails. Not sure how this happened - it just started showing up. Can this be deleted (and how?)
This isn't being added by Marketo but by your mailserver.
Speak to your IT about the specific characteristics they're using to determine whether to add this header to inbound mail (mail that uses your domain, but is sent from outside your corporate infrastructure).
That's controlled by your spam server. Not much to do. Ask IT to whitelist your IP and domains, but that won't work for anyone you actually email to.