I believe this is due in part to the Marketo sending domains having a consistent identifiable string (mkto) that is picked up by many enterprise email filters. I'm not sure that this can be avoided or otherwise manipulated, but I'd also be very interested in finding out.
As Eric Stone mentions, "mktoemail.com" could be considered as a "promotional" element in the email. Using a branded subdomain could affect marketing efforts positively. There are various best practices to follow and try to get into primary emails. For example,
various sources mention that, Gmail would push anything which looks like promotion, to promotions tab. (Looking for more ideas here is this should also apply to outlook)
The following are the practices which we can deploy, and hope for the best chance of getting the primary tab -
1. Addressed to an actual name (using tokens)
2. Minimum number of external links
3. Avoid heavy use of images
4. Write the content copy in natural, rather than promotional language - hence avoiding the words like, "Discount", "Free", "Coupon" etc.
5. Use an email address which looks like an email address of a person in From/Reply sections. Looking for more ideas here.