When Contact Pruning is enabled, contacts are automatically blocked after a specified number of emails are sent with 0 opens.
The Contact Pruning setting is on your Settings Screen under the Sending tab.
Set a limit on the number of emails that can be sent to a contact with out any opens.
Contact that go over this limit are then set to a "Stale" status in the account.
"Stale" contacts will be suppressed from receiving non-transactional email. Transactional email can still be sent to Stale contacts.
To send to contacts that have opted into transactional email only, you must:
A) HTTP API - Pass the parameter "isTransactional = true"
B) SMTP Relay - Add a header "IsTransactional" with the value of "True"