This flag is applied according to bounce parameters can be seen under:
Core> Communication> Bounces>  Parameters

We recommend setting Soft Bounces to: 8
Hard bounces cannot be changed from: 1

Additionally, these are defined as:
  • Soft Bounce: An email message gets as far as the recipient's mail server but bounces back undelivered before it gets to the intended recipient.
    Examples: recipient's inbox is full, delivery time expired, connection problems.
  • Hard Bounce: An email is bounced back to the sender as undeliverable without being accepted by the recipient's mail server.
    Examples: invalid e-mail address (syntax error, recipient is unknown, domain name doesn't exist)