- Bounce messages look like gibberish but tell you exactly what went wrong.
- Learn to read hard and soft bounces, the codes that matter, and when to remove an address for good.
Every bounced email is a diagnosis waiting to be read. Most people delete bounce notifications in annoyance; smart senders read them, because bounce handling is one of the biggest levers on sender reputation. Here is how to decode them.
Hard Bounces: Permanent Failures
A hard bounce means the message can never be delivered to that address. SMTP codes start with 5 (permanent):
- 550 No such user — the mailbox does not exist; the person left the company or the address was mistyped.
- Domain not found — the domain expired or was misspelled.
- 550 rejected by policy — the receiver refuses your server, often a blacklist or authentication failure rather than a bad address.
Action: remove genuinely dead addresses from your lists immediately. Repeatedly mailing dead mailboxes is how spam-trap hits happen and how reputations die.
Soft Bounces: Temporary Failures
A soft bounce means delivery failed now but may succeed later. Codes start with 4 (temporary):
- Mailbox full — extremely common with quota-limited cPanel mailboxes in Nepal.
- Server temporarily unavailable — the receiving server is down or overloaded.
- Greylisting — the receiver deliberately defers first-time senders; the retry succeeds minutes later.
- Message too large — attachment exceeds the receiver limit.
Action: mail servers retry automatically for hours or days. If an address soft-bounces across several separate campaigns, treat it as dead.
Reading a Bounce Message
- Find the three-digit code — 4xx temporary, 5xx permanent.
- Read the text after it; phrases like user unknown, quota exceeded, or listed at name the exact cause.
- Check which server generated the bounce — your own server bouncing means a local problem (quota, authentication); the remote server bouncing means a recipient-side or reputation problem.
Bounce Rate and Your Reputation
Inbox providers watch the percentage of your mail that hard-bounces. Above roughly 2%, you look like a sender with a scraped or ancient list, and filtering tightens across all your mail. Keep rates low by:
- Removing hard bounces immediately and automatically.
- Using double opt-in on signup forms to stop typo addresses entering lists.
- Re-confirming lists older than a year before mailing them.
- Never buying or scraping addresses.
When Your Own Mail Bounces Everywhere
Sudden bounces to many different recipients is not a list problem — it is your server: blacklisting, broken SPF/DKIM after a DNS change, or a full outgoing queue. Treat it as an incident, and see our blacklist removal guide for the recovery steps.
Frequently Asked Questions
Do bounces cost reputation even when they are not my fault?
Hard bounces count against you regardless of fault — that is why list hygiene is a sender responsibility.
Why did mail bounce with over quota from a customer?
Their mailbox is full. Phone or message them another way; the mail is not waiting, it is gone unless your server retries.
What is a bounce loop?
Two auto-responding systems bouncing at each other. Modern servers suppress these, but misconfigured forwarders can still create them — avoid forwarding chains.
Our managed email plans monitor bounce patterns for you — or send us a confusing bounce message and we will decode it. See also corporate hosting for high-volume senders.
