Hard bounce: permanent email delivery failure

Hard bounce is a permanent delivery refusal from the recipient's mail server. The server tells the sender the message cannot be delivered and retrying will not help. The address must be removed from the mailing list.

Causes of a hard bounce

The most common cause: the mailbox does not exist. Someone changed jobs, the corporate account was deleted, or a subscriber typed their address wrong and no one caught it.

• Non-existent mailbox. The server responds with code 550 or 553. Typical messages: “User unknown”, “Mailbox not found”, “No such user”.
• Non-existent domain. DNS finds neither an MX nor an A record. The message never reaches an SMTP connection.
• Domain that refuses mail. The MX record points to a null host (RFC 7505), explicitly blocking all incoming email.
• Blocked address. The server permanently rejects mail to this mailbox: code 550 with “permanently rejected”.
• Addressing error. A format that passed client-side validation but was rejected by the server.

SMTP codes for hard bounces

SMTP uses three-digit response codes. Codes starting with 5 mean permanent failures:

• 550 - requested action not taken, mailbox unavailable.
• 551 - user not local; please try a different path.
• 552 - storage quota exceeded (sometimes a soft bounce, depending on the provider).
• 553 - requested action not taken, mailbox name syntax error.
• 554 - transaction failed. Usually accompanied by a server policy explanation.

Enhanced Status Codes (RFC 3463) give more detail: 5.1.1 - mailbox does not exist, 5.1.2 - domain does not exist, 5.2.1 - mailbox disabled.

Why a high hard bounce rate is a problem

Mail providers track the bounce rate of every sender. A hard bounce rate above 2% is a red flag. Gmail and other major providers start routing that sender's messages to spam for all recipients, not just those whose addresses bounced.

ESPs (email service providers) react too. Mailchimp automatically pauses a campaign when the bounce rate crosses its threshold. SendGrid and others can suspend a sender account. Reputation takes weeks or months to recover.

How to handle hard bounces

The only correct response: remove the address immediately. Not later, not after one more attempt. Sending again to an address that hard-bounced makes things worse. The provider can see the sender is ignoring rejections.

Most ESPs do this automatically: after a hard bounce the address goes into a suppression list and is excluded from all future campaigns. If you manage sending yourself via your own MTA or an SMTP relay, bounce processing is your responsibility.

How to prevent them

Hard bounces are easier to prevent than to recover from. Three approaches that actually work:

1. Validate at collection. Real-time email verification on the signup form catches non-existent addresses before they enter your database.
2. Double opt-in. The confirmation email only reaches live mailboxes. If the subscriber never clicks the link, the address stays inactive.
3. Regular list cleaning. Batch validation every 3-6 months. Addresses go stale over time and only a fresh check shows their current status.