Hard bounce

A hard bounce is a permanent delivery failure — the receiving server has told you in no uncertain terms that the message will never be delivered, usually because the address does not exist or the domain cannot receive mail.

What it is

SMTP servers return one of three response classes. 2xx means accepted, 4xx means try again later — that's a soft bounce. 5xx means stop, do not retry, the message will never be accepted. Anything in that 5xx range is a hard bounce. The most common are 550 5.1.1 User unknown, 550 5.1.10 Recipient address rejected, and 550 5.7.1 Message blocked.

The address either does not exist on that domain, the domain itself has no working mail server, or the receiver has decided your mail will never be accepted regardless of contents.

Common causes

Most hard bounces are unknown-user errors: a typo at signup, an employee who left the company, an address scraped from a stale source. Some are domain-level: the company has shut down, lost the domain, or misconfigured its MX records. A growing share are policy-based: the receiver has classified you as a spammer and rejects everything from your IP or domain with a 550.

The distinguishing feature is permanence. Whatever the cause, the receiver is telling you that no retry will succeed.

The must-remove rule

There is exactly one correct response to a hard bounce: add the address to a suppression list and never send to it again. Every reputable ESP does this automatically because the alternative is reputation suicide. Re-sending to a known bad address tells mailbox providers that you do not curate your list — the single strongest signal of a spammer in their internal models.

If you import a list into a new tool, carry your hard-bounce suppressions with you. Forgetting this step is one of the most common ways established senders torpedo a new sending domain in the first week.

Thresholds that matter

Gmail's bulk-sender rules from February 2024 made a 0.3 percent complaint rate the explicit ceiling. Bounce rate is less formal but the practical thresholds are well-known. Under 2 percent campaign-wide is safe. Between 2 and 5 percent you'll see throttling and increased spam placement. Above 5 percent and Gmail, Microsoft, and Yahoo will start actively suppressing your mail.

For cold email specifically, where lists are inherently less clean, the right move is to verify every address with a real-time validation service before the first send. Verification removes most of the unknown-user bounces before they ever generate a 550, which keeps your ratio in the safe band.

Why it matters

Hard bounces are the single most expensive signal a sender can emit — they're a direct admission that the list is bad. A spike in hard bounces on a new domain you're still warming can wipe out weeks of positive engagement signal in a single afternoon. Keep them off the list and warmup can do its job.

Related

• Soft bounce — the temporary counterpart
• Spam trap — what some hard bounces actually were
• Sender reputation — the score bounces hammer
• Cold email deliverability checklist
• NeverSpam pricing