Bounce message analyzer: decode any email bounce code or NDR
Paste the bounce email, the NDR or just its error line. The analyzer finds the SMTP reply code (550) and the enhanced status code (5.1.1), says whether it's a hard or soft bounce and whether the problem is the address or your sending, and links each email bounce code to its causes and fixes. Nothing you paste leaves your browser.
Sample result
550 5.1.1 is a hard bounce: jane@acme.example was rejected.
- Bounce code
- 550 5.1.1
- Recipient
- jane@acme.example
- Action
- Remove the address
What this means for you
Remove the address; sending to it again hurts your sender reputation.
- Finds basic and enhanced SMTP codes
- Sorts hard, soft and policy bounces
- Pulls recipients from DSN fields
- Falls back to wording when no code is found
- Bounces
- SMTP codes
- DSN
By SimpleVerifier Team · Last updated · Runs in your browser; nothing you paste is sent to us
How do I read an email bounce message?
- Open the bounce, the email titled "Undeliverable", "Mail delivery failed" or "Delivery Status Notification (Failure)", and copy all of it (the first 500,000 characters are read). Or upload it as a .eml file.
- Paste it into the box and press Analyze bounce.
- Read the verdict: a bad address, a mailbox problem, a policy block, or a temporary failure.
- Follow the link for the code's causes and fixes. If you paste a log with many bounces, the table counts each code.
Example
- You enter
- A pasted bounce containing
550 5.1.1 <jane@acme.example>: Recipient address rejected. - You get
- A hard bounce for a bad address:
jane@acme.examplewas rejected, so remove it rather than sending again. - Limits
- Runs in your browser; the first 500,000 characters of the pasted text are read.
What do email bounce codes mean?
A bounce code says whether the failure is permanent or temporary and what it was about, and most bounces carry two of them. The three-digit reply code comes from RFC 5321: 4xx means a temporary failure the sending server should retry, 5xx a permanent one. The enhanced status code, like 5.1.1, comes from RFC 3463 and is more specific. Its first number repeats the class (4 or 5). Its second is the subject: 1 is the address, 2 the mailbox, 4 routing, 7 security or policy. The third is the detail.
So 550 5.1.1 reads as: permanent failure, addressing problem, the mailbox doesn't exist. 452 4.2.2 is a temporary failure because the mailbox is full. 550 5.7.1 is permanent, but it's a policy decision about your message or your server, not proof that the address is bad. Our SMTP error code reference lists each common code with real examples.
What does SMTP error 550 mean?
SMTP error 550 means the receiving server permanently refused the message for that recipient; the enhanced code after it says why. The common ones:
550 5.1.1: the mailbox doesn't exist, a hard bounce. 550 5.4.1: Microsoft 365's "recipient address rejected, access denied", which in practice behaves like 5.1.1. 550 5.7.1: refused by policy, often reputation or a block list. 550 5.7.26: Gmail refused unauthenticated mail. 550 5.7.515: Outlook.com refused a high-volume domain that fails its authentication rules.
"550 relay access denied" is different again. It means the server you handed the message to won't pass it on, usually because your mail client is pointed at the wrong outgoing server or isn't authenticating with it. The recipient's address may be fine.
Is it a hard bounce, a soft bounce or a block?
Look at the class and the subject: 5.1.x is a hard bounce, any 4xx is a soft bounce, and 5.7.x is usually a block. A hard bounce means the address won't work: it doesn't exist, the domain doesn't exist, or the mailbox is disabled. Remove it and don't retry, because repeated sends to dead addresses are what mailbox providers count against you.
A soft bounce (4xx) is temporary: a full mailbox, a busy server or greylisting. Your sending server retries on its own. Suppress the address only if it keeps failing over several days. See hard bounce vs soft bounce for how the main platforms treat each.
A policy block (5.7.x) is the one people mishandle. The address is often fine; the receiving server refused the message because of authentication (SPF, DKIM, DMARC), your IP's reputation, a block list, or the content. Deleting the address doesn't help. Fixing the cause does. Our bulk sender checker covers the authentication side.
Where is the error code in a bounce message?
It depends on who sent the bounce. Microsoft 365 and Outlook.com put it under "Diagnostic information for administrators", after "Remote Server returned". Gmail's bounce quotes the receiving server's reply after "The response was:". Standard delivery status notifications, defined in RFC 3464, carry Status: and Diagnostic-Code: fields in a machine-readable part that the analyzer reads, along with Final-Recipient, the address that failed. Our guides to Gmail bounce codes and bounce messages by mailbox provider show real examples.
If there's no code at all, the analyzer falls back to the wording, such as "user unknown" or "mailbox full", and says that it matched on words rather than a code.
Hard bounce, soft bounce or block: what should I do?
| Hard bounce | Soft bounce | Policy block | |
|---|---|---|---|
| Typical codes | 550 5.1.1, 553 5.1.2, 550 5.2.1 | 452 4.2.2, 421 4.7.0, 451 4.3.0 | 550 5.7.1, 554 5.7.1, 550 5.7.26 |
| Will retrying work? | No | Usually; your server retries | Not until the cause is fixed |
| Remove the address? | Yes | Only after repeated failures | Usually not |
| What to fix | The list | Nothing, or wait | Authentication, reputation or content |
Who uses this tool, and for what?
- Cold emailers
- Paste a batch of bounces from your outreach tool to see how many are dead addresses (
5.1.x) and how many are blocks (5.7.x). Many 5.1.1s mean the list needs verifying; many 5.7.x mean the domain or IP does. - Sales ops
- Decide which CRM contacts to mark as bad. Only hard bounces should be suppressed; a policy block or a full mailbox is not a reason to delete a lead.
- Marketers
- Read the bounce report from a campaign, then work out what the bounces cost with the bounce rate calculator.
- IT admins
- Decode an NDR a user forwards from Outlook or Gmail, and tell them in one line whether it's a typo, a full mailbox or a block on your own server.
What can’t this tool tell you?
- It reads only what's in the text you paste. A bounce that says nothing beyond "delivery failed" can't be diagnosed further.
- Providers word bounces differently and sometimes use codes loosely. When no code is found, the verdict is a match on wording and says so.
- A
5.7.xcode says the message was refused by policy, not which policy. The rest of the server's text, if any, is the only clue. - Codes your email platform invents, such as its own numbered bounce reasons, are not SMTP codes and aren't decoded.
Common questions
- What does 550 5.1.1 mean?
- The receiving server says the mailbox doesn't exist. It's a permanent failure and a hard bounce: remove the address and don't retry it. If many addresses on the same list return it, the list is stale and the rest should be verified before the next send.
- What are the most common email bounce codes?
- 550 5.1.1 (mailbox doesn't exist), 550 5.4.1 (Microsoft's recipient rejected), 552 5.2.2 or 452 4.2.2 (mailbox full), 550 5.7.1 and 554 5.7.1 (refused by policy or as spam), and 421 4.7.0 (temporarily deferred). Each has a page in our SMTP code reference with causes and fixes.
- What is the difference between a hard and a soft bounce?
- A hard bounce (5xx, especially 5.1.x) is permanent: the address or domain doesn't work, so remove it. A soft bounce (4xx) is temporary, such as a full mailbox or a busy server, and your server retries it automatically.
- What does SMTP error 554 mean?
- The transaction failed permanently. In bounces it most often appears as 554 5.7.1, a rejection as spam or by policy, so treat it like a 5.7.x block: find the cause in the message text rather than deleting the address.
- What does 550 relay access denied mean?
- The server you sent through refused to forward your message. Check that your mail client or app uses the right outgoing (SMTP) server for your account and authenticates with it. It is not a verdict on the recipient's address.
- Should I delete an address that returned 550 5.7.1?
- Usually not. 5.7.x codes are policy rejections about your message, IP or domain, such as failed authentication or a block list. The address is often valid. Fix the cause the message names and retry.
- What does an "Undeliverable" email mean?
- Your message couldn't be delivered, and the receiving or sending server sent back a non-delivery report (NDR) explaining why. The code inside it says whether the address is bad, the mailbox is full, or the message was blocked.
- Is the bounce I paste stored anywhere?
- No. The analyzer runs in your browser and only reads the text you paste. Nothing is sent to our servers.
Sources
A tidy list can still be full of dead addresses.
Extracting, deduplicating and syntax checks all work on the text of an address. None of them can tell whether the mailbox exists. Only the server that hosts it can answer that, through an SMTP check, and that is what the paid product does.
- Upload the cleaned file as it is: CSVs up to 100,000 rows
- A real SMTP mailbox check on every row, with catch-all detection
- Small list? $9 one-time covers 2,500 emails, no subscription
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