A bounce is an error message that is automatically generated by the recipient server when an e-mail cannot be delivered. There is a difference between a hard bounce and a soft bounce.

• The hard bounce is a permanent problem, e.g. that the address no longer exists. Addresses that generate a hard bounce are immediately unsubscribed by promio.connect and are not contacted again.
• By contrast, a soft bounce is a temporary problem, e.g. a recipient’s absence during vacation or a full mailbox. The soft bounce rate is also an indicator of the address quality.

Bounces are marked in the data field “mailerror”:

• 0: delivered
• 1: hard bounce
• 2: soft bounce
• 3: quarantine