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 without any opens.

Contact that goes over this limit is 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"