Hard bounce
A hard bounce is a permanent email delivery failure indicating that a message cannot be delivered and will not be deliverable on any future attempt. Hard bounces typically occur because the email address does not exist, the domain is invalid or no longer active, or the recipient’s mail server has permanently blocked the sender. Unlike soft bounces, which are temporary failures that may resolve on retry, a hard bounce signals a permanent problem that requires immediate action.
Hard bounces are one of the most direct signals of list quality. A high hard bounce rate indicates that your contact database contains a significant proportion of invalid, outdated, or fabricated addresses. This is most commonly the result of importing old lists, purchasing contact data, or failing to validate email addresses at the point of collection. Even a small percentage of hard bounces on every send compounds damage to your sender reputation over time, as inbox providers interpret them as a sign that you are not managing your list responsibly.
For B2B marketers, suppressing hard bounces immediately and permanently is a non-negotiable list hygiene practice. Most email service providers do this automatically after the first hard bounce. The harder operational challenge is preventing new hard bounces from entering your list in the first place: using double opt-in at signup, running email validation on imported lists, and maintaining regular database hygiene are the most effective preventative measures.
What causes a hard bounce?
The most common causes are an email address that never existed or was entered incorrectly, an address that has been deactivated because the person has left a company, a domain that no longer exists or has expired, and a receiving server that has permanently blocked the sender’s IP or domain. Each of these indicates a permanent barrier to delivery that cannot be resolved by retrying the send.
What is the difference between a hard bounce and a soft bounce?
A hard bounce is a permanent delivery failure: the address is definitively unreachable and should never be attempted again. A soft bounce is a temporary failure: the address exists and the server accepts mail, but the specific delivery attempt failed due to a full mailbox, a server timeout, or a message size issue. Soft bounces can be retried. Hard bounces should be suppressed immediately and permanently.
What hard bounce rate is acceptable?
A hard bounce rate below 0.5 percent per send is generally considered healthy. Rates above 2 percent indicate a significant list quality problem that needs addressing before you run further campaigns. Most email service providers have thresholds above which they will pause sends or require account review. Keeping hard bounce rates consistently low requires ongoing list hygiene: validating new addresses at collection, removing hard bounced contacts promptly, and regularly auditing the database for obvious invalid formats.