SMTP codes · RFC 5321 · RFC 3463
SMTP error codes
Paste the error message you got (a bounce, a non-delivery report) and see what each code says, how Gmail and Microsoft 365 document it and what to do. Or pick a code from the list below.
How to read an SMTP code
- The basic code: three digits
The first digit gives the outcome:
2success,3more input expected,4temporary failure,5permanent failure (RFC 5321 §4.2.1). The second digit gives the category (x5zis the mail system), the third refines it. - The enhanced code: class.subject.detail
A code like
5.7.26follows the basic one (RFC 3463): the class (2,4,5) repeats the outcome, the subject (1address,2mailbox,3system,4network,5protocol,6content,7security and policy) says what it is about, and the detail says exactly what. The official list is the IANA registry. - Temporary or permanent
With
4xxthe sending server retries on its own for a while; there is no need to resend by hand. With5xxresending the same message does not help: something about the address, the message or the server must change. - The non-delivery report (DSN)
Servers send back a report with standard fields (RFC 3464):
Action(failed, delayed…),Status(the enhanced code),Remote-MTA(the server that refused) andDiagnostic-Code(its exact reply). The last one is the most useful.
All codes
The basic codes of RFC 5321, then the enhanced codes of the IANA registry and those Gmail and Microsoft 365 document, by subject.
Basic codes
211System status214Help message220Service ready221Service closing channel235Authentication succeeded250Action completed251User not local, will forward252Cannot verify user, will accept354Start mail input421Service not available432Password transition needed450Mailbox unavailable (temporary)451Local error in processing452Insufficient system storage454Temporary failure (TLS or authentication)455Unable to accommodate parameters500Command unrecognized501Syntax error in parameters502Command not implemented503Bad sequence of commands504Parameter not implemented521Server does not accept mail530Authentication required534Authentication mechanism too weak535Authentication credentials invalid538Encryption required for the mechanism550Mailbox unavailable (permanent)551User not local552Exceeded storage allocation553Mailbox name not allowed554Transaction failed555MAIL/RCPT parameters not recognized556Domain does not accept mail
X.0.YYY · No specific subject
No additional subject information is available.
5.0.0Undefined status5.0.350Generic error, x-dg-ref header is too long , or Requested action not taken: policy violation detected (AS345)
X.1.YYY · Address
About the originator or destination address: syntax or validity. The sender can usually correct it and retry.
5.1.0Address problem5.1.1Recipient mailbox does not exist5.1.2Recipient domain cannot receive mail5.1.3Recipient address syntax is invalid5.1.4Recipient address is ambiguous2.1.5Recipient address is valid5.1.6Mailbox moved, no forwarding address5.1.7Sender address syntax is invalid5.1.8Sender domain cannot receive mail5.1.9Relayed to a non-compliant mailer5.1.10Recipient domain has a null MX5.1.20Multiple From addresses are not allowed without Sender address5.1.90Your message can't be sent because you've reached your daily limit for message recipients5.1.241The message can't be delivered because the recipient is an HVE account
X.2.YYY · Mailbox
Something about the recipient's mailbox; under the recipient's control.
5.2.0Mailbox problem5.2.1Mailbox disabled5.2.2Mailbox full5.2.3Message too big for the mailbox5.2.4Mailing list expansion problem5.2.121Recipient's per hour message receive limit from specific sender exceeded5.2.122Recipient's per hour message receive limit exceeded5.2.240The email account is not set up properly for the HVE service5.2.241The HVE account is not properly set up for billing
X.3.YYY · Mail system
Something about the destination server; under its administrator's control.
5.3.0Mail system problem4.3.1Mail system full5.3.2System not accepting messages5.3.3Selected features not supported5.3.4Message too big for the system5.3.5System incorrectly configured2.3.6Requested priority was changed5.3.190Journaling on-premises messages to Microsoft 365 or Office 365 not supported when Journaling Archive is disabled4.3.240The maximum number of concurrent server connections has exceeded a per authenticated source limit, closing transmission channel
X.4.YYY · Network and routing
Network and routing status (DNS included); under the control of the destination's or an intermediate server's administrator.
5.4.0Network or routing problem5.4.1No answer from host4.4.2Bad connection4.4.3Directory server failure5.4.4Unable to route5.4.5Mail system congestion5.4.6Routing loop detected5.4.7Delivery time expired5.4.8MX hosts of <domain> failed MTA-STS validation5.4.14Routing loop detected4.4.244The HVE message can't be submitted because the recipient rate limit was exceeded5.4.300Message expired4.4.316Connection refused [Message=Socket error code 10061]4.4.317Cannot connect to remote server [Message=UntrustedRoot]
X.5.YYY · Protocol
Failures of the delivery protocol: implementation errors or an unreliable connection.
5.5.0Protocol problem5.5.1Invalid command5.5.2Syntax error5.5.3Too many recipients5.5.4Invalid command arguments5.5.5Wrong protocol version5.5.6Authentication line too long
X.6.YYY · Content
Problems with the message content (types, conversions); up to both sender and recipient.
5.6.0Content or media problem5.6.1Media not supported5.6.2Conversion required and prohibited5.6.3Conversion required but not supported2.6.4Conversion with loss performed5.6.5Conversion failed5.6.6Message content not available5.6.7Non-ASCII addresses not permitted5.6.8UTF-8 reply required but not permitted5.6.9UTF-8 header cannot be transferred5.6.11Invalid characters5.6.240One of MIME Headers is not allowed5.6.241Mandatory From Header is missing or invalid
X.7.YYY · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
5.7.0Security or policy problem5.7.1Delivery not authorized, message refused5.7.2Mailing list expansion prohibited5.7.3Security conversion not possible5.7.4Security features not supported5.7.5Cryptographic failure5.7.6Cryptographic algorithm not supported5.7.7Message integrity failure5.7.8Authentication credentials invalid5.7.9Authentication mechanism too weak5.7.10Encryption needed5.7.11Encryption required for the mechanism5.7.12Password transition needed5.7.13User account disabled5.7.14Trust relationship required5.7.15Priority level too low5.7.16Message too big for the priority5.7.17Mailbox owner has changed5.7.18Domain owner has changed5.7.19RRVS test cannot be completed5.7.20No passing DKIM signature5.7.21No acceptable DKIM signature5.7.22No author-matched DKIM signature5.7.23SPF validation failed5.7.24SPF validation error5.7.25Reverse DNS validation failed5.7.26Multiple authentication checks failed5.7.27Sender address has a null MX5.7.28Mail flood detected5.7.29ARC validation failure5.7.30REQUIRETLS support required5.7.32Your email was blocked because the From: header (RFC5322) in this message isn't aligned with either the authenticated SPF or DKIM organizational domain5.7.40Your message was blocked because the sending domain doesn't have a DMARC record or the DMARC record doesn't specify a DMARC policy5.7.57Client was not authenticated to send anonymous mail during MAIL FROM5.7.64TenantAttribution; Relay Access Denied5.7.80Username and Password not accepted5.7.90Application-specific password required5.7.124Sender not in allowed-senders list5.7.133Sender not authenticated for group5.7.134Sender was not authenticated for mailbox5.7.135Sender was not authenticated for public folder5.7.136Sender was not authenticated5.7.142XOAUTH2 authentication failed5.7.143XOAUTH2 authentication failed5.7.144XOAUTH2 authentication failed5.7.232Your message can't be sent because your trial tenant has exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit)5.7.233Your message can't be sent because your tenant exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit)5.7.236Your message can't be sent because your tenant has exceeded its daily limit for sending email to external recipients from your tenant's onmicrosoft.com domains5.7.240The application is not allowed for use with an HVE account5.7.241Trial tenant is not able to use HVE5.7.242Inactive tenant is not able to use HVE5.7.243This tenant is blocked from using HVE5.7.244Message rejected5.7.245External sending is not supported for HVE accounts5.7.246Removed external recipients that were not originally specified on submission5.7.321starttls-not-supported: Destination mail server must support TLS to receive mail5.7.322certificate-expired: Destination mail server's certificate is expired5.7.323tlsa-invalid: The domain failed DANE validation5.7.324dnssec-invalid: Destination domain returned invalid DNSSEC records5.7.325certificate-host-mismatch: Remote certificate MUST have a common name or subject alternative name matching the hostname (DANE)5.7.367Remote server returned not permitted to relay4.7.500-699Access denied, please try again later5.7.501Access denied, spam abuse detected5.7.502Access denied, banned sender5.7.503Access denied, banned sender5.7.504[email@contoso.com]: Recipient address rejected: Access denied5.7.505Access denied, banned recipient5.7.506Access Denied, Bad HELO5.7.507Access denied, rejected by recipient5.7.508Access denied, [$SenderIPAddress] has exceeded permitted limits within $range range5.7.509Access denied, sending domain [$SenderDomain] does not pass DMARC verification and has a DMARC policy of reject5.7.510Access denied, [contoso.com] does not accept email over IPv65.7.511Access denied, banned sender5.7.512Access denied, message must be RFC 5322 section 3.6.2 compliant5.7.513Service unavailable, Client host [$ConnectingIP] blocked by $recipientDomain using Customer Block list (AS16012607)5.7.520Access denied, Your organization does not allow external forwarding5.7.606-649Access denied, banned sending IP [IP1.IP2.IP3.IP4]5.7.703Your message can't be delivered because messages to XXX, YYY are blocked by your organization using Tenant Allow Block List5.7.705Access denied, tenant has exceeded threshold5.7.708Access denied, traffic not accepted from this IP5.7.750Service unavailable5.7.800Access denied, banned sender4.7.850-899Access denied, please try again later
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026