Christina, we also experience that some of our clients block Marketo's IPs. This isn't necessarily specific to Marketo, they also block Listrak, Mailgun, etc. So you might be able to communicate with people one minute using your company's IP, and then the next minute an email from Marketo will soft-bounce.
Unfortunately, I'm not sure if there is a very good solution for this, other than asking customer to whitelist the Marketo IPs where your sends originate from.