Hard Bounce

A hard bounce is a rejection message from a receiving mail server indicating a permanent problem with delivery. The SMTP reply is in the 5xx range, and the enhanced status code starts with 5, such as 5.1.1 for "bad destination mailbox address".

Why it matters

Repeatedly sending to addresses that do not exist is a signal of a poorly maintained list, and mailbox providers use bounce rates to judge senders. Many email platforms suppress addresses after the first hard bounce for this reason. Keeping the bounce rate low protects deliverability for everyone on your domain or IP.

Example

A rejected message returns something like:

550 5.1.1 The email account that you tried to reach does not exist.

Typical permanent causes: the mailbox does not exist, the domain has no mail server, or the receiver has blocked your sending IP or domain. Some policy blocks, such as a blacklist listing, use 5xx codes too, so read the text before deleting the subscriber.

Common mistakes

• Treating every 5xx as an invalid address. A block due to your reputation is not the recipient's fault.
• Retrying hard bounces, which wastes sending reputation.
• Importing old lists without verification.

Related

• Soft bounce
• Bounce rate
• Sender reputation
• गाइड: Email bounce codes explained

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