Deliverability
Updated
Hard bounce
Definition
A hard bounce is a permanent delivery failure: the receiving server rejected the email and retrying will not help. The most common cause is an address or domain that does not exist.
How it works
Receiving servers signal permanent failures with SMTP reply codes in the 5xx range, often with an enhanced status code (RFC 3463) that explains why. 5.1.1 means the mailbox does not exist. 5.1.2 points to a bad destination domain.
Not every 5xx is about the address. Codes in the 5.7.x range usually mean a policy rejection, such as failed authentication or a blocklisted IP. Those need a sending fix, not a list fix.
Your ESP receives the bounce either during the SMTP session or as a non-delivery report sent to the Return-Path address, then marks the address as undeliverable.
Example
550 is a permanent failure. 5.1.1 means the mailbox does not exist.
Why it matters
Repeatedly mailing addresses that do not exist is one of the clearest signals of a bad list. It hurts sender reputation and can trigger spam filtering for everyone on your list.
Abandoned addresses can be turned into recycled spam traps. Continuing to mail a hard-bounced address risks hitting one later.
Best practices
- Suppress an address after the first hard bounce for an invalid mailbox or domain. Never re-import it.
- Read the code. A 5.7.x policy block calls for fixing authentication or reputation, not deleting the subscriber.
- Catch typos at signup (gmial.com, hotmial.com) with double opt-in or address verification.
- Keep an eye on hard bounces per import source. One bad CSV can explain most of them.
How Lumail handles hard bounce
Lumail suppresses addresses automatically after a permanent bounce, across campaigns, workflows and transactional sends, so they are not mailed again.
Blocklist and policy rejections are surfaced as their own deliverability alert, separate from bad addresses, with a dedicated fix guide.
Check a real message with the free mail tester or the spam tester.
Frequently asked questions
What is the difference between a hard and soft bounce?
A hard bounce is permanent (5xx), for example a mailbox that does not exist. A soft bounce is temporary (4xx), for example a full mailbox or a throttled server, and the sending server retries.
Can a hard-bounced address become valid again?
Occasionally, if a mailbox is recreated. It is rarely worth the risk. If the person signs up again themselves, that is the safe way back.
Why did a valid address hard bounce?
Check the status code. 5.7.x codes are policy rejections such as failed DMARC or a blocklisted sending IP. The address is fine. Fix authentication or reputation instead.
Related terms, tools and docs
- DeliverabilitySoft bounceA soft bounce is a temporary delivery failure: the receiving server deferred the email but the address may accept it later.
- MetricsBounce rateEmail bounce rate is the percentage of sent emails that the receiving server rejected as undeliverable.
- List managementList hygieneList hygiene is the routine of removing or suppressing email addresses that hurt deliverability: hard bounces, spam complainers, long-inactive subscribers, invalid addresses and likely spam traps..
- DeliverabilitySender reputationSender reputation is the trust score mailbox providers assign to your sending domain and IP addresses, based on how recipients and their systems react to your mail.
- Free toolFree mail testerSend a real email and get an SPF, DKIM, DMARC and content report.
- DocsHow to lower bounce rateWhat to do when permanent bounces pass 2% or 5%.
- DocsHow to fix blocklist bouncesPolicy rejections that are not bad addresses.
- DocsEmail verification APICheck an address before you add it.
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