You see a record in the database has something similar to the following Email Suspended Cause:
"554- Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means."
The 554 error is generated by a hard bounce due to a spam block. It is possible that one of the sending IP addresses used may have been on a temporary blacklist the day of the attempted send. This happens while using shared IPs when another Marketo instance using the same IP hits a spam trap, putting the IP on a blacklist for 24 hours. This error can also happen to users on a dedicated IP if they hit a spam trap with one of their email sends.