Are you using a dedicated IP?
If not, Marketo Deliverability handles blacklists for shared IPs. Not much to do about it. But you could send a ticket in.
We have been getting the following messages lately.
“522 email sent from (snsmtp.mktomail.com) found on industry URI blacklist, please contact your mail sending service or use an alternative email service to send your email. Guide for bulk senders www.bt.com/bulksender”
I’ll tell the team to send a ticket next time it happens.
if that's a shared IP, then Deliverability Team should handle it. You may not be able to do much about it yourself.