Contact Cadence: this is up to you. Some ecommerce or news sites could send several times a day. Some firms have explicit statements like no more than 2 promos/mo to buy a product (webinars don't count). I personally think that I shouldn't send out more than 2 emails/week. Now if you tell someone up front that they will get an email every day, that's ok, they are expecting that. If you think you need to throttle down, then go to once a week, especially for promo emails. Check your programs and unsub rates to see which ones might be irritating people.
Deliverability - if you just switched to Marketo (I think you did), then you need to warm up your IP (if you have one) and setup DKIM/SPF properly. If you can avoid changing email addresses and domains, that's also good. If you got caught by a blacklist, call Support to help you resolve the issue.
Also check you are using your own branded links for tracking and not the default Marketo links! Secondly strip down your templates and make them super email client proof, run them through SpamAssasin and EmailonAcid to check your spam- and reputation score.
These and the things Josh mentioned helped us. We now have 99-100% delivery rate in our engagement program mailings.