SMTP error codes explained
SMTP reply codes starting with 5 are permanent failures — the message will never be delivered and must not be retried. Codes starting with 4 are temporary, and the sending server will retry automatically. The digits after the first one narrow down the reason, and that is where the fix lives.
Cleaning a list after a bounce? The common codes are marked hard bounce, soft bounce or block, and each of those pages says whether to remove the address, with Gmail, Microsoft or Yahoo wording quoted where the provider publishes it. Microsoft and Outlook.com errors, including 550 5.7.515, are grouped on the Microsoft and Outlook hub. Every enhanced status code in the IANA registry, and the codes Gmail, Microsoft 365 and Yahoo document themselves, are in the enhanced status code index. To read a whole bounce message, use the bounce message analyzer.
Permanent failures — 5xx
Do not retry these. Continuing to send to a permanent failure is one of the fastest ways to damage sender reputation.
- 550 5.1.1Hard bounce
mailbox does not exist
SMTP 550 5.1.1 means the recipient's mail server accepted the connection but rejected the address: the mailbox does not exist on that domain.
- 550 5.7.1Block
message refused by policy
SMTP 550 5.7.1 means the recipient's server recognised the address but refused the message on policy grounds — failed authentication, a blocklisted sending IP, or content its filters rejected.
- 550 5.4.1Hard bounce
recipient address rejected, access denied
SMTP 550 5.4.1 is Microsoft 365's rejection for an address it does not recognise or will not accept mail for.
- 552 5.2.2Hard bounce
mailbox full
SMTP 552 5.2.2 means the recipient's mailbox is over its storage quota and the server will not accept the message.
- 554 5.7.1Block
transaction failed, message rejected as spam
SMTP 554 5.7.1 is a flat refusal: the receiving server has decided the message is spam, or that your sending IP or domain is not permitted to send to it.
- 550 5.2.1Hard bounce
mailbox disabled, not accepting messages
SMTP 550 5.2.1 means the mailbox exists but has been disabled and will not accept mail.
- 503 5.5.1
bad sequence of commands
SMTP 503 5.5.1 means the commands arrived in an order the server does not accept — for example DATA before RCPT TO, or a command issued before authentication.
- 535 5.7.8
authentication credentials invalid
SMTP 535 5.7.8 means the username or password offered during authentication was rejected.
- 571 5.7.1
delivery not authorized, message refused
SMTP 571 5.7.1 is a policy refusal: the receiving server will not accept your message and will not retry.
- 500
syntax error, command unrecognised
SMTP 500 means the receiving server did not recognise the command it was sent.
- 501
syntax error in parameters or arguments
SMTP 501 means the command was recognised but its parameters were not valid.
- 502
command not implemented
SMTP 502 means the server recognises the command but does not support it.
- 503
bad sequence of commands
SMTP 503 means a command was sent in the wrong order.
- 504
command parameter not implemented
SMTP 504 means the server understood the command but does not support one of its parameters.
- 521
host does not accept mail
SMTP 521 means the host you connected to does not accept mail at all.
- 530
authentication or encryption required
SMTP 530 means the server will not go further until you authenticate or start TLS.
- 535
authentication credentials invalid
SMTP 535 means the server rejected the credentials your client offered.
- 541
message rejected by the recipient's filter
SMTP 541 is not defined in RFC 5321.
- 550
mailbox unavailable
SMTP 550 means the requested action was not taken because the mailbox is unavailable.
- 551
user not local
SMTP 551 means the recipient is not hosted on this server.
- 552
exceeded storage allocation
SMTP 552 means the server aborted the action because of a storage limit.
- 553
mailbox name not allowed
SMTP 553 means the server will not accept the mailbox name.
- 554
transaction failed
SMTP 554 means the transaction failed.
- 555
MAIL FROM or RCPT TO parameters not recognised
SMTP 555 means the server did not recognise or does not support a parameter on MAIL FROM or RCPT TO.
- 550 5.1.0
address rejected, other address status
SMTP 550 5.1.0 means the receiving server rejected the address without giving a more specific reason.
- 553 5.1.2Hard bounce
recipient domain not found
SMTP 553 5.1.2 means the domain in the recipient address does not exist or cannot accept mail.
- 553 5.1.3Hard bounce
invalid address syntax
SMTP 553 5.1.3 means the recipient address is not a valid email address.
- 551 5.1.6
mailbox has moved
SMTP 551 5.1.6 means the recipient's mailbox has moved and the server is not forwarding mail to it.
- 550 5.1.10Hard bounce
recipient not found (Microsoft 365)
SMTP 550 5.1.10 is Exchange Online's "recipient not found" error: Microsoft looked up the recipient's address and found no mailbox for it.
- 550 5.1.8Block
bad outbound sender (Microsoft 365)
SMTP 550 5.1.8 means Microsoft 365 has blocked your own account from sending.
- 552 5.3.4Block
message too big
SMTP 552 5.3.4 means the message is larger than the receiving server accepts.
- 550 5.4.5Block
daily sending limit exceeded (Gmail)
SMTP 550 5.4.5 is Google's "daily user sending limit exceeded".
- 554 5.4.4
unable to route, domain not found
SMTP 554 5.4.4 means the sending server could not find a route to the recipient's domain.
- 550 4.4.7Soft bounce
message expired in the queue
4.4.7 means your message sat in the sending queue until it expired, so the sender gave up.
- 550 5.7.25Block
no PTR record for sending IP (Gmail)
SMTP 550 5.7.25 means Gmail rejected the message because the sending IP has no PTR (reverse DNS) record, or the PTR does not match a forward DNS record.
- 550 5.7.26Block
blocked, sender unauthenticated (Gmail)
SMTP 550 5.7.26 is Gmail rejecting mail that failed authentication.
- 550 5.7.27Block
SPF authentication failed (Gmail)
SMTP 550 5.7.27 means Gmail blocked the message because it did not pass SPF.
- 550 5.7.28Block
unusual rate of unsolicited mail (Gmail)
SMTP 550 5.7.28 means Gmail is blocking mail from your IP because it has seen an unusual rate of unsolicited email from it.
- 530 5.7.0
must issue STARTTLS or authenticate first
SMTP 530 5.7.0 means the server needs your client to encrypt the connection with STARTTLS, or to authenticate, before it will accept mail.
- 530 5.7.57
client not authenticated (Microsoft 365)
SMTP 530 5.7.57 is Microsoft 365 refusing mail from a device or app that connected to smtp.office365.com without signing in.
- 550 5.7.23Block
SPF violation (Microsoft 365)
SMTP 550 5.7.23 is Microsoft 365 rejecting a message because of an SPF failure.
- 550 5.7.124Block
sender not in allowed senders list (Microsoft 365)
SMTP 550 5.7.124 means you sent to a Microsoft 365 distribution group that only accepts mail from listed senders, and you are not on that list.
- 550 5.7.509Block
DMARC failed, policy reject (Microsoft 365)
SMTP 550 5.7.509 means Microsoft 365 rejected the message because the domain in its From header failed DMARC, and that domain publishes p=reject.
- 550 5.7.606Block
banned sending IP (Microsoft)
SMTP 550 5.7.606 to 5.7.649 means Microsoft has put your sending IP on its block list.
- 550 5.7.705
tenant has exceeded threshold (Microsoft 365)
SMTP 550 5.7.705 means Microsoft has blocked outbound mail from your whole Microsoft 365 tenant.
- 550 5.7.708
traffic not accepted from this IP (Microsoft 365)
SMTP 550 5.7.708 means Microsoft 365 will not relay mail from your tenant's sending IP because that IP has a low reputation.
- 550 5.7.750
blocked from sending from unregistered domains (Microsoft 365)
SMTP 550 5.7.750 means Microsoft 365 has blocked your tenant because it sent a large amount of mail from domains that are not registered as accepted domains in the tenant.
- 550 5.7.515Block
authentication required for high-volume senders (Outlook.com)
SMTP 550 5.7.515 is Outlook.com rejecting mail from a high-volume sender that does not meet its authentication requirements.
- Outlook.com S3140 (550 5.7.1)
IP range on Outlook.com block list
Outlook.com error S3140 is a 550 5.7.1 rejection meaning part of your sending network is on Microsoft's consumer block list.
- 554 5.6.0Block
message malformed
SMTP 554 5.6.0 means the receiving server rejected the message because its content or structure is malformed.
- 550 5.5.0Hard bounce
Mailbox unavailable (Outlook.com, Hotmail)
550 5.5.0 "Requested action not taken: mailbox unavailable" is what Outlook.com returns when a Hotmail, Outlook or Live address does not exist.
- 554 5.4.14Block
Routing loop detected
554 5.4.14 means the message went round in a loop between mail servers until Microsoft stopped it.
- 550 5.7.12Block
Sender not authenticated by organization (Microsoft 365)
550 5.7.12 means the recipient's Microsoft 365 address only accepts mail from inside its own organisation, so mail from outside is rejected.
- 550 5.7.513Block
Blocked by the recipient's own block list (Microsoft 365)
550 5.7.513 means the recipient's organisation added your sending IP to its own block list in Microsoft 365.
- 552 5.7.0Block
Blocked for a security risk in the content (Gmail)
552 5.7.0 from Gmail means the message was blocked because its content, usually an attachment, presents a potential security issue.
- Yahoo TSS09Block
Yahoo permanent deferral of your IP
TSS09 is a Yahoo rejection that senders report as a permanent deferral of all mail from their IP, with a note that retrying will not succeed.
- Yahoo 554 delivery errorHard bounce
This user doesn't have a yahoo.com account
"554 delivery error: dd This user doesn't have a yahoo.com account" is Yahoo's hard bounce for an address that does not exist.
Temporary deferrals — 4xx
These resolve on their own in most cases. A 4xx during mailbox verification is not a verdict on the address — it means unconfirmed.
- 421 4.7.0Soft bounce
temporarily deferred, try again later
SMTP 421 4.7.0 is a temporary deferral: the recipient's server is refusing the message for now and asking you to retry.
- 451 4.3.0Soft bounce
temporary local error, please retry
SMTP 451 4.3.0 is a temporary failure on the receiving side — a local processing error, a full queue, or a filtering component that could not reach a service it depends on.
- 250
accepted, which is not the same as delivered
SMTP 250 means the receiving server accepted the command.
- 450 4.2.1Soft bounce
mailbox temporarily unavailable
SMTP 450 4.2.1 means the mailbox exists but cannot accept mail right now — commonly because it is disabled, over quota, or the server is rate-limiting you.
- 452 4.2.2Soft bounce
insufficient storage, mailbox full
SMTP 452 4.2.2 means the recipient's mailbox is over its storage quota, returned as a temporary failure so the sending server retries.
- 421
service not available, closing channel
SMTP 421 means the receiving server is not available right now and is closing the connection.
- 450
mailbox unavailable, try again later
SMTP 450 means the requested action was not taken because the mailbox is unavailable for now.
- 451
local error in processing
SMTP 451 means the receiving server aborted the request because of an error on its side.
- 452
insufficient system storage
SMTP 452 means the receiving server will not take the message right now because of a storage or quota limit.
- 451 4.4.2
connection timed out
SMTP 451 4.4.2 means the connection dropped or timed out partway through the transaction.
- 421 4.7.26Soft bounce
rate limited, unauthenticated (Gmail)
SMTP 421 4.7.26 is Gmail rate limiting mail because it is not authenticated.
- 421 4.7.28Soft bounce
unusual rate of mail from your IP (Gmail)
SMTP 421 4.7.28 means Gmail has seen an unusual rate of mail from your IP or domain and is deferring it to protect its users.
- 451 4.7.500Soft bounce
access denied, try again later (Microsoft 365)
SMTP 451 4.7.500 to 4.7.699 (shown with an ASxxx tag) is Microsoft 365 temporarily throttling mail from your IP.
- 451 4.7.1Soft bounce
Temporarily deferred (greylisting or policy)
451 4.7.1 means the receiving server has deferred your message for now, most often because of greylisting or a temporary policy check.
- 421 4.4.5Soft bounce
Server busy
421 4.4.5 means the receiving server is too busy to take your message right now and closed the connection.
- Yahoo TSS04Soft bounce
Yahoo temporary deferral
TSS04 is a Yahoo temporary deferral, sent with a 421 4.7.0 reply.
Reading a reply code
Every SMTP reply has a three-digit basic code and, usually, a three-part enhanced code. The basic code’s first digit is the only part that decides whether to retry: 2xx accepted, 4xx temporary failure, retry, and 5xx permanent failure, do not retry.
The enhanced code adds precision. In 5.1.1 the first digit repeats the class, the second names the subject — 1 for address problems, 7 for policy and security — and the third is the specific detail. So 5.1.1 is a permanent address problem, and 5.7.1 is a permanent policy refusal. They need completely different fixes, which is why the enhanced code matters.
Always read the text after the code too. Most mail servers name the actual reason in plain language, and many include a URL to their own policy page.
Most of these are preventable
550 5.1.1 is the code you see when an address does not exist, and it is the one verification eliminates entirely — before the bounce is recorded against your domain rather than after. The policy codes, 5.7.1 and 554, usually come down to SPF, DKIM and DMARC on your sending domain.
- Verify your list before you send — plans from $9 a month, see pricing
- Verify at signup with the API — stop bad addresses entering the list
- Audit your sending domain — MX, SPF, DKIM, DMARC, blocklists
- Deliverability glossary — the terms behind these codes