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.
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?