Please see my slides and guide on this.http://www.slideshare.net/jdavidhill/marketo-email-reputation-management
Deliverability depends on a lot of things. Your new Tool (not IBM) is about to be released, so use that to run some campaigns or email templates through. Fix anything it flags.
>>SPF is meaningless for Marketo emails, despite the hype
Why?
Because emails from Marketo don't rely on your SPF record, but on Marketo's (for the envelope sender <verp@*.mktomail.com>).
More info here: Re: Mailed By em-sj-77.mktomail.com - How to Fix?