Hard bounces: the number that quietly damages your sender reputation

A bounce is what happens when an email couldn't be delivered, and the receiving server sends a notice back explaining why. Not all bounces mean the same thing, and mailbox providers care a great deal about the difference. A soft bounce is temporary — the recipient's inbox was full, or their server was briefly unreachable — and it's expected to eventually succeed if retried. A hard bounce is permanent: the address doesn't exist, the domain doesn't exist, or the mailbox was closed for good. There's no retry that fixes a hard bounce.

When a receiving server rejects a message, it replies with a 3-digit SMTP status code, and almost always an extended status code alongside it (RFC 3463) in the form class.subject.detail. The leading digit of both is what actually separates permanent from temporary:

These arrive inside a formal Delivery Status Notification (DSN) message, standardized by RFC 3464. A DSN's machine-readable part carries an Action: field — failed for a hard bounce, delayed for a soft one — alongside the Status: code itself. Reading those two fields directly, rather than trying to interpret the human-readable explanation text, is what makes it possible to tell a genuine hard bounce apart from a temporary one reliably.

• A simple typo in the email address that was never caught before sending.
• An account that was closed or an employee who left the company long ago.
• A domain that no longer exists at all.
• Addresses from an old, purchased, or unverified list that was never checked for accuracy.

Mailbox providers treat a sender's hard bounce rate as one of the clearest available signals of list quality. A sender who keeps hitting dead addresses is either buying stale lists, scraping addresses carelessly, or simply not verifying who they're emailing — and all three patterns correlate strongly with spam behavior, even when the sender's actual intentions are perfectly legitimate.

There's a widely cited industry rule of thumb: keep a hard bounce rate under roughly 2%. Cross that line consistently, and most mailbox providers start treating the sending pattern as a genuine reputation risk rather than a minor, forgivable list-quality issue — which can quietly drag down inbox placement for every message that sender sends afterward, not just the ones that bounced.

• Verify email addresses before sending to them for the first time, rather than after the fact.
• Periodically clean existing lists to remove addresses that have started bouncing.
• Avoid old, purchased, or scraped lists — they decay in quality far faster than most people expect.
• Stop emailing addresses that bounce even once as a hard bounce; retrying them doesn't help and only makes the pattern worse.