Bounce codes
Email bounce codes: what each SMTP error means
Got an email bounce back? Look up the SMTP code: what it means, whether it's your list or your domain, and how to fix it. Gmail, Outlook and Yahoo.
How to read a bounce code
Click any code below for its full page: the provider's exact wording, whose problem it is and how to fix it.
An email bounce back carries two codes. The first, like 550, is the basic reply: 2 means accepted, 4 means try again later and 5 means rejected for good. The second, like 5.7.1, says why:
- X.1.X, addressing: something about the address itself: unknown, wrong or badly formed.
- X.2.X, mailbox: the recipient's mailbox: full, disabled or inactive.
- X.4.X, network and routing: the delivery path between the servers.
- X.7.X, security or policy: a policy refused the message: authentication, reputation, filtering or a setting on their side.
- X.0.X, other: nothing more specific. The text after the code is all you have.
So 550 5.1.1 and 550 5.7.1 share a first code but are opposite problems: a dead address versus a server that refused you. Always read the second code and the text after it.
Hard bounce vs soft bounce
A hard bounce is a permanent failure (a 5xx code): the address doesn't exist, or the server refuses your mail. Sending again won't help, so stop. A soft bounce is temporary (a 4xx code): a full mailbox, a busy server, a rate limit. Servers retry those on their own. A run of soft bounces from the same provider is still a warning about how it sees you.
Every code, at a glance
| Code | What it means | Usually |
|---|---|---|
| 550 5.7.1 | 550 5.7.1 means the receiving server refused your message on policy grounds. The text after the code says why: spam, reputation, a block list or a setting. Gmail, Microsoft 365 / Outlook, Any server | Depends on the text |
| 550 5.1.1 | 550 5.1.1 means the recipient's mailbox doesn't exist: a typo, a deleted account or a guessed address. It's a list problem. Remove the address and move on. Gmail, Microsoft 365 / Outlook, Yahoo, Any server | List problem |
| 550 5.4.1 | Despite the wording, Microsoft's 550 5.4.1 "Access denied" means the recipient address doesn't exist in that organization. It's a list problem, not a block. Microsoft 365 / Outlook | List problem |
| 550 5.7.26 | Gmail's 550 5.7.26 means your message failed authentication: SPF and DKIM both failed, or your domain's DMARC policy said to reject it. Fix your DNS records. Gmail | Sender problem |
| 421 4.7.28 | Gmail's 421 4.7.28 means it's temporarily rate-limiting you: it saw an unusual rate of mail from your IP or domain. Slow down; it's an early warning. Gmail | Sender problem |
| 421 4.7.0 | Gmail's 421 4.7.0 is a temporary refusal with several causes: low IP or domain reputation, suspicious links, a missing PTR record, required TLS. Read the text. Gmail | Depends on the text |
| 554 5.7.1 | 554 5.7.1 is a permanent policy refusal: the transaction failed and the sender wasn't authorized. The reason is in the text after the code. Any server | Depends on the text |
| 550 5.0.350 | Microsoft's 5.0.350 wraps an error the recipient's server returned, so it isn't the real reason. Find the original code inside the bounce and handle that. Microsoft 365 / Outlook | Depends on the text |
| 550 5.1.10 | Microsoft's 550 5.1.10 means it couldn't find the recipient's address. Usually it doesn't exist; sometimes a broken forwarding rule on their side. Microsoft 365 / Outlook | List problem |
| 550 5.7.509 | Microsoft's 550 5.7.509 means your From domain failed DMARC and its policy is reject, so Microsoft refused the message. Fix SPF or DKIM alignment. Microsoft 365 / Outlook | Sender problem |
| 550 5.7.511 | Microsoft's 550 5.7.511 means the IP you're sending from is banned. Fix what got it banned, then ask for delisting at [email protected]. Microsoft 365 / Outlook | Sender problem |
| 550 5.7.708 | Microsoft's 550 5.7.708 means it banned sending from your tenant because most of its traffic looked suspicious. Fix the cause, then contact Microsoft support. Microsoft 365 / Outlook | Sender problem |
| 451 4.7.500 | Microsoft's 4.7.500 means it saw suspicious activity in your sending and restricted it for a while. If the activity is legitimate, it lifts on its own. Microsoft 365 / Outlook | Sender problem |
| 554 5.7.9 | Yahoo's 554 5.7.9 means the message wasn't accepted for policy reasons, typically authentication: the sending domain isn't using DMARC, or fails it. Yahoo | Sender problem |
| 550 5.7.25 | Gmail's 550 5.7.25 means the IP that sent your message has no reverse DNS (PTR) record. Add one that matches, or have your provider set it. Gmail | Sender problem |
| 550 5.2.1 | 550 5.2.1 means the mailbox exists but isn't accepting mail; Gmail uses it for inactive accounts. For outreach, treat it as a dead address. Gmail, Any server | List problem |
| 552 5.2.2 | Gmail's 552 5.2.2 means the mailbox is full and the account inactive, so it's a hard bounce. For outreach, treat the address as dead. Gmail | Their mailbox |
| 452 4.2.2 | 452 4.2.2 means the recipient's mailbox is out of space. It's temporary and on their side: your server retries, and your domain isn't at fault. Gmail, Any server | Their mailbox |
| 450 4.2.1 | Gmail's 450 4.2.1 means that one recipient is getting mail faster than Gmail allows, so it's deferring yours. It's temporary; space out your sends. Gmail | Their mailbox |
| 553 5.1.3 | 553 5.1.3 means the recipient address isn't a valid email address at all: a stray character, a missing @ or a broken import. Fix it or remove it. Gmail, Any server | List problem |
| 550 5.7.520 | Microsoft's 550 5.7.520 isn't a spam block: the forwarding mailbox's own organization blocks automatic forwarding to outside addresses. Microsoft 365 / Outlook | Their mailbox |
| 550 5.7.133 | Microsoft's 550 5.7.133 means you emailed a group (distribution list) that only accepts mail from inside its organization. Only their admin can change it. Microsoft 365 / Outlook | Their mailbox |
| 550 5.7.134 | Microsoft's 550 5.7.134 means the recipient's mailbox only takes mail from authenticated, usually internal, senders. Only their admin can change it. Microsoft 365 / Outlook | Their mailbox |
Seeing a lot of bounces at once? High email bounce rate? Burned domain or bad list walks through telling the two apart.