As inboxes are getting smarter, it's getting tougher and tougher go get in front of people via email. As Vladislav Vagner mentioned, Outlook web has a "Focussed" and "Other" inherently designed to weed out "Marketing" emails from real people. You see this too from Gmail/Google Apps where it auto-sorts into social, updates, promotions, etc.
From what I've read, the pattern seems to be messages with "Unsubscribe" in the email, as well as amount of images and HTML used in the email body are showing up in these auto sorted tabs more and more and not ever getting in front of user's eyes.
While there are some things you can do to the email (such as cutting down on the graphics and HTML), ultimately cold email is likely to continue showing less and less positive results. As others have alluded to, getting people to engage through quality content is going to yield the best results, but is also the hardest to do. My advice would be to keep focussing on creating relevant content that is easy to find (SEO), and be active in the communities where your prospects tend to hang out such as LinkedIn, or other Social sites (however, I would caution that if you only share "Look at me!" content, it will just add to the noise on these platforms).
Anyway - I know this was a little beyond the scope of your initial question, but hopefully helps your team going forward.
Best of luck!
One of the biggest issues that I see for customers is the content and quality of the emails. I generally ignore open rates since they aren't as reliable of an email metric. Therefore, I look more to click-through-ratio and custom conversations (such as did they fill out the form you wanted on the page where you directed them?)
Was the content compelling enough for your audience to want to open the email? Did it address a pain point? Was there a solid reason - for the customer - to click through the email? If it was overly salesy or unclear about what the reader gets by clicking through, then you won't get good results.
People's inboxes are crowded so you have to show your audience that you understand them and offer them value. That includes offering them value for the time it takes to open, read and click an email.
For new Marketo instances you would want to start to "warm" the instance by only sending to customers, or people that know your company so they actually engage with the email and your sender score can be raised and be more trusted similar to what Amit Jain suggested. Something you can do when someone fills out a form is remind them to look for your domain in their email inbox and add you to their address book, or ask the sales team to remind customers to add the domain to their address book so your company can communicate important updates.
Purchasing lists is bad practice and against Marketo policy, as mentioned above, and can not only hurt your sender score/Marketo's shared IPs and annoy the person you're emailing, it also gives your brand a bad name too. With the new GDPR-like laws being voted on or implemented in America, purchasing data even from DiscoverOrg is going to become a much dicier situation.
If you're sending to people who have opted in or are customers, your best bet would be to send a few emails as text-only (no images, little-to-no styling) and make sure your subject lines don't have spam trigger words (the finance industry has several- I usually will check this list before sends: The Ultimate List of Email SPAM Trigger Words)