Thanks guys.
I've been informed this seems to be an issue with our company's email spam filter blocking Marketo's IP address.
Sample emails - as well as campaign emails - sent to our company email addresses aren't being received. Anything sent to personal email addresses (Hotmail & Yahoo for example) are fine.
Our head of Digital has been notified & we're looking into it from our side with our IT dept, so leave it with us.
Cheers,
Steve.