Bounced Email

When delivery of an email message fails, the mailbox provider sends our email system a code combination of numbers and text to indicate a soft or hard failure. We organize these codes into failure reason categories to help you understand and correct the problem.

• A hard bounce occurs when a mailbox provider rejects an email message, such as if the recipient email address does not exist.

  Note: To ensure a positive sender reputation, our system automatically suppresses email addresses that return hard bounces from future mailings. For more information, see Suppression Lists.

• A soft bounce occurs when an email message is processed by the recipient's mailbox provider but is returned as undeliverable before it reaches the recipient.

• A block failure occurs when an email recipient's mailbox provider intentionally refuses a message, such as if the sending IP address is on an external blacklist or the SPAM complaint rate for the sender is high.

• A technical failure occurs when delivery to an address or domain fails due to difficulties that occur during the attempt to connect with the recipient's mailbox provider.