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)
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