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It looks like the domain used in the email's From address doesn't have DKIM set up. DKIM is an authentication method you want to set up so that the recipient's email server can verify that Marketo has permission to send email on behalf of your company. It prevents people from pretending to be at your company to steal information. Since it's not set up, it look's that recipient is outright rejecting the message because it appears suspicious.
Go to Admin > Email, click the SPF/DKIM tab, and add your domain:
Follow the instructions here for how to work with your IT team to get it set up:
Actually it sounds like the DKIM setup is broken, not that it's unconfigured. This is really bad: a broken signature is one of the few ways a recipient server can be completely sure a message is bad and must not be delivered.
I'd recommend making sure DKIM is immediately disabled for this Marketo account, then starting over and setting it up right.