| Error Code | Meaning | Troubleshooting | Glossary |
| 421, “4.3.0” | Temporary System Problem. Try again later | Retry sending the message later | SMTP: Simple Mail Transfer Protocol, a protocol for sending e-mail messages |
| 421, “4.4.5” | The server is busy; try again later | Retry sending the message later | – |
| 421, “4.7.0” | IP not in whitelist for RCPT domain, closing connection | Check if your IP address is allowed to send emails to the receiver | RCPT: SMTP command to specify the recipient of the message |
| 421, “4.7.0” | Our system has detected an unusual rate of unsolicited mail originating from your IP address. | Review your sent emails for any potential unsolicited mail. | – |
| 421, “4.7.0” | Temporary System Problem. Try again later | Retry sending the message later | – |
| 421, “4.7.0” | TLS required for RCPT domain, closing connection | Make sure your system supports TLS (Transport Layer Security) | TLS: Protocol that provides communications security over a computer network |
| 421, “4.7.0” | Try again later, closing the connection | Retry sending the message later | DoS: Denial of Service, a cyber-attack that makes a machine or network resource unavailable |
| 450, “4.2.1” | The user you are trying to contact is receiving mail too quickly | Resend your message at a later time | – |
| 450, “4.2.1” | The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. | Resend your message at a later time. | – |
| 450, “4.2.1” | Peak SMTP relay limit exceeded for customer | Try reducing the number of sent emails | SMTP relay: Process of transferring an email from one mail server to another |
| 451, “4.3.0” | The mailMail server temporarily rejected the message | Retry sending the message later | – |
| 451, “4.3.0” | Multiple destination domains per transaction is unsupported | Send your message to a single domain at a time | Domain: Part of the network address that represents a specific entity (like a company) on the internet |
| 451, “4.4.2” | Timeout – closing connection | Retry sending the message later | – |
| 451, “4.5.0” | SMTP protocol violation | Ensure that your system is correctly following the SMTP protocol | SMTP: Simple Mail Transfer Protocol, a protocol for sending e-mail messages |
| 452, “4.2.2” | The email account that you tried to reach is over quota | Ask the recipient to clear their inbox | Over quota: When a user has filled their storage limit |
| 452, “4.5.3” | Domain policy size per transaction exceeded | Try sending to the recipient in a separate transaction | – |
| 452, “4.5.3” | Your message has too many recipients | Reduce the number of recipients for your message | – |
| 454, “4.5.0” | SMTP protocol violation, no commands allowed to pipeline after STARTTLS. | Check your SMTP relay settings and try again. | STARTTLS: A way to take an existing insecure connection and upgrade it to a secure connection using SSL/TLS. |
| 454, “4.7.0” | Cannot authenticate due to a temporary system problem. | Try again later. If the problem persists, contact your email provider. | Authenticate: To verify the identity of a user, device, or system. |
| 501, “5.5.2” | Cannot decode the response. | This can be a server-side issue. Try again later or contact your service provider. | Decode: To convert from an encoded or encrypted form to plain text. |
| 501, “5.5.4” | HELO/EHLO argument is invalid. | Check your SMTP server configuration for errors. | HELO/EHLO: SMTP commands sent by an email server to identify itself when connecting to another email server. |
| 502, “5.5.1” | Too many unrecognized commands, goodbye. | Check your SMTP commands for any mistakes. | SMTP Commands: Instructions that an email client communicates to the mail server to send an email. |
| 502, “5.5.1” | Unimplemented command. | Ensure that your email client is using commands recognized by the SMTP server. | Unimplemented: Not yet put into effect. |
| 503, “5.5.1” | “EHLO/HELO first. | Your email client may be out of order. Ensure it starts with an EHLO or HELO command. | EHLO/HELO: Commands used by SMTP to initiate the connection. |
| 503, “5.5.1” | MAIL first. | The MAIL command needs to be the first command in your SMTP conversation. | MAIL: An SMTP command is used to initiate mail transfer. |
| 503, “5.5.1” | RCPT first. | The RCPT command needs to be the first command in your SMTP conversation. | RCPT: An SMTP command is used to identify the email’s recipient. |
| 503, “5.7.0” | No identity changes are permitted. | Check your SMTP configuration or contact your service provider. | Identity: The unique identifier (often a username or email address) associated with a specific user. |
| 504, “5.7.4” | Unrecognized Authentication Type | Check your SMTP settings and ensure you’re using a supported authentication type. | Authentication Type: The method used to verify a user’s identity, such as password or OAuth. |
| 530, “5.5.1” | Authentication Required | Check that you’re properly authenticated with your SMTP credentials. | Authentication: The process of verifying a user’s identity. |
| 530, “5.7.0” | Must issue a STARTTLS command first | Ensure that your email client initiates the SMTP connection with a STARTTLS command. | STARTTLS: A way to take an existing insecure connection and upgrade it to a secure connection using SSL/TLS. |
| 535, “5.5.4” | Optional Argument not permitted for that AUTH mode | Check your SMTP settings, especially the AUTH command and its arguments. | AUTH: An SMTP command is used to authenticate the client to the server. |
| 535, “5.7.1” | Application-specific password required | Use an application-specific password if you have 2-Step Verification enabled. | Application-specific password: A password for applications not supporting logging in using 2-Step Verification. |
| 535, “5.7.1” | Please log in with your web browser and then try again | Log into your account via your web browser, then resend your email. | – |
| 535, “5.7.1” | Username and Password not accepted | Ensure that you’re entering the correct username and password. If you’re using 2-Step Verification, use an application-specific password. | – |
| 550, “5.1.1” | The email account that you tried to reach does not exist | Check the recipient’s email address for typos or unnecessary spaces. | – |
| 550, “5.2.1” | The email account that you tried to reach is disabled | You cannot send an email to a disabled account. Contact the recipient through other means if possible. | – |
| 550, “5.2.1” | The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered | Try again later. | – |
| 550, “5.4.5” | Daily sending quota exceeded | Limit the number of emails you send each day or upgrade your Google Workspace edition if you need to send more mail. | Quota: A set limit on how much of a particular resource a user or program can use. |
| 550, “5.4.5” | Daily SMTP relay limit exceeded for user | Check your SMTP relay settings or upgrade your Google Workspace edition to send more mail. | SMTP relay: A service used to send large volumes of email. |
| 550, “5.7.0” | Mail relay denied | The email was not sent because the SMTP relay was denied; contact your service provider. | Relay: The act of passing email from one server to another. |
| 550, “5.7.0” | Mail Sending denied | Ensure the sender account is active and part of your Google Workspace domain. | – |
| 550, “5.7.1” | Email quota exceeded | You’ll need to free up storage space or purchase more in order to send additional emails. | Quota: A set limit on how much of a particular resource a user or program can use. |
| 550, “5.7.1” | Invalid credentials for relay | Check the configuration of your SMTP relay service. | Credentials: Information that includes identification and proof of identification used to gain access to resources. |
| 550, “5.7.1” | Our system has detected an unusual rate of unsolicited mail originating from your IP address. | If you’re not sending spam, examine your network for malware or misconfigured software. | Unsolicited mail: Email that a user did not ask for (usually refers to spam). |
| 550, “5.7.1” | Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to Gmail, this message has been blocked | Ensure the email complies with Gmail’s Bulk Email Senders Guidelines | Unsolicited mail: Email that a user did not ask for (usually refers to spam). |
| 550, “5.7.1” | The IP you’re using to send mail is not authorized to send email directly to our servers | Use the SMTP relay at your service provider instead | SMTP relay: A service used to send large volumes of email. |
| 550, “5.7.1” | The user or domain that you are sending to (or from) has a policy that prohibits the mail that you sent | Review the email content and contact the domain administrator for further details. | Policy: A rule or set of rules governing how systems or organizations operate. |
| 550, “5.7.1” | Unauthenticated email is not accepted from this domain | Check your SMTP settings and ensure the email passes SPF or DKIM authentication | Unauthenticated email: An email that hasn’t passed validation checks to prove it came from the domain it claims to be from. |
| 550, “5.7.1” | Daily SMTP relay limit exceeded for customer | Check your SMTP relay settings or upgrade your Google Workspace edition if you need to send more mail | SMTP relay: A service used to send large volumes of email. |
| 550, “5.7.26” | Unauthenticated email from domain name is not accepted due to domain’s DMARC policy. | Adjust your DMARC records, consider using a service like Google Workspace to send your mail, and contact the administrator of the receiving mail server. | DMARC: An email validation system that detects and prevents email spoofing. |
| 550, “5.7.26” | This message does not have authentication information or fails to pass authentication checks. | Ensure the email passes SPF or DKIM authentication | Authentication: The process of verifying the identity of a user, process, or device. |
| 550, “5.7.26” | This message fails to pass SPF checks for an SPF record with a hard fail policy. | Adjust your SPF records and ensure the email passes SPF authentication | SPF (Sender Policy Framework): An email authentication method designed to detect forging sender addresses during email delivery. |
| 552, “5.2.2” | The email account that you tried to reach is over quota | Ask the recipient to clear some space in their inbox | Quota: A set limit on how much of a particular resource a user or program can use. |
| 552, “5.2.3” | Your message exceeded Google’s message size limits | Reduce the size of the message and try again | – |
| 553, “5.1.2” | We weren’t able to find the recipient domain | Double-check the recipient’s email address for typos or unnecessary spaces | Domain: The part of an email address that comes after the @ symbol. |
| 554, “5.6.0” | The mail message is malformed. Not accepted | Check the format of your email and try again | Malformed: Structured incorrectly or in a way that does not conform to the expected format. |
| 554, “5.6.0” | Message exceeded 50 hops; this may indicate a mail loop | Check your SMTP settings to prevent potential mail loops | Mail Loop: A situation in which an email message is continually bounced back and forth between two or more servers. |
| 554, “5.7.0” | This message was blocked because its content presents a potential security issue. | Ensure the content and attachments of the email comply with Google’s policies. | – |
| 554, “5.7.0” | IP has been blacklisted due to spam activity | Contact your service provider and take steps to remove your IP from the blacklist. | Blacklist: A list of entities blocked or denied access to a certain system or protocol. |
| 554, “5.7.1” | The transaction failed because the recipient’s mailbox is full | Ask the recipient to clear some space in their mailbox | Mailbox: An electronic folder where incoming emails are stored for a user. |
| 554, “5.7.1” | The message you’re trying to send exceeds the recipient’s size limits | Reduce the size of the message or contact the recipient to increase their size limits | Size limits: Restrictions on the maximum size of an email message or attachment. |
| 555, “5.5.2” | Syntax error | Check the syntax of the email addresses, commands, or configuration settings. | Syntax: The set of rules that dictate the structure and format of a language or code. |
| 555, “5.5.4” | Optional Argument not permitted for that AUTH mode | Review your SMTP authentication settings and ensure they are correctly configured | Authentication: The process of verifying the identity of a user, process, or device. |
| 555, “5.7.1” | Authentication required for relay | Provide valid authentication credentials for the SMTP relay | Relay: Forwarding email messages from one mail server to another. |
| 555, “5.7.1” | Please log in with your web browser and then try again | Access your Gmail account via a web browser and follow any on-screen prompts to resolve the issue | Web browser: A software application used to access and view websites. |
| 555, “5.7.1” | Username and Password not accepted | Verify the correctness of your username and password, and ensure they are correctly entered. | Username: A unique identifier used to access a system or service. |
| 550, “5.1.1” | The email account that you tried to reach does not exist | Double-check the recipient’s email address for typos, or contact the recipient to confirm the correct email address | Typos: Errors or mistakes in the spelling or formatting of text. |
| 550, “5.2.1” | The email account that you tried to reach is disabled | Verify the status of the recipient’s email account or contact the recipient to confirm the account status | Disabled: Inactive or deactivated. |
| 550, “5.4.5” | Daily sending quota exceeded | Reduce the number of outgoing emails or upgrade your Google Workspace edition for higher sending quotas. | I am sending quota: The maximum number of emails allowed to be sent within a specified time period. |
| 550, “5.4.5” | Daily SMTP relay limit exceeded for user | Check your SMTP relay sending limits or contact your administrator for assistance | SMTP relay: A service used to send large volumes of email. |
| 550, “5.7.0” | Mail relay denied | Check your SMTP relay settings and ensure they comply with Google’s policies. | Mail relay: Forwarding email messages from one mail server to another. |
| 550, “5.7.0” | Mail Sending denied. This error occurs if the sender account is disabled or not registered within your Google Workspace domain. | Verify the sender’s email account status and ensure it is enabled and registered within the Google Workspace domain. | Domain: A group of computers or devices that share a common set of rules and settings. |
| 550, “5.7.1” | Email quota exceeded | Reduce your mailbox size or upgrade your Google Workspace edition for higher storage quotas. | Quota: The allocated amount or limit for a particular resource or service. |
| 550, “5.7.1” | Invalid credentials for relay | Verify the correctness of your authentication credentials for the SMTP relay. | Credentials: Information used to verify one’s identity or access a system. |
| 550, “5.7.1” | Our system has detected an unusual rate of unsolicited mail originating from your IP address. | Check your network for malware or misconfigured software to prevent spamming activities. | Unsolicited mail: Email that is sent without the recipient’s explicit consent. |
| 550, “5.7.1” | Our system has detected that this message is likely unsolicited mail | Ensure your email content and attachments comply with Google’s spam policies to prevent message blocking | Spam: Unsolicited or unwanted email, often sent in bulk. |
| 550, “5.7.1” | The IP you’re using to send mail is not authorized to send emails directly to our servers. | Use the SMTP relay provided by your service provider to send an email or contact Google support for further assistance | SMTP: Simple Mail Transfer Protocol, the standard protocol used for email transmission. |
| 550, “5.7.1” | The user or domain that you are sending to (or from) has a policy that prohibits the mail that you sent | Review and comply with the policies of the recipient’s user or domain to ensure successful email delivery. | Policies: Rules or guidelines that govern the behavior or actions within a system or organization. |
| 550, “5.7.1” | Unauthenticated email is not accepted from this domain | Configure your email client or server to authenticate before sending emails | Authentication: The process of verifying the identity or legitimacy of a user or entity. |
| 550, “5.7.1” | Daily SMTP relay limit exceeded for customer | Review your SMTP relay sending limits or contact your administrator for assistance. | SMTP relay: A service used to send large volumes of email. |
| 550, “5.7.26” | Unauthenticated email from domain name is not accepted due to domain’s DMARC policy | Authenticate your email with the appropriate mechanisms specified by the DMARC policy of the recipient’s domain. | DMARC: Domain-based Message Authentication, Reporting, and Conformance, a security protocol for preventing email spoofing. |
| 550, “5.7.26” | This message does not have authentication information or fails to pass authentication checks (SPF or DKIM) | Ensure your email passes SPF and DKIM authentication checks to improve deliverability. | SPF: Sender Policy Framework, a protocol that validates the origin of email messages. DKIM: DomainKeys Identified Mail, a method for validating the authenticity of email messages. |
| 552, “5.2.2” | The email account that you tried to reach is over quota | Advise the recipient to free up space in their mailbox by deleting unnecessary emails | Quota: The allocated storage capacity for an email account. |
| 552, “5.2.3” | Your message exceeded Google’s message size limits | Reduce the size of your email message or use alternative methods for sharing large files | Message size limits: The maximum allowed size for an email message, including attachments. |
| 553, “5.1.2” | We weren’t able to find the recipient domain | Verify the correctness of the recipient’s email address and check for any typos or formatting errors. | Domain: The part of an email address after the “@” symbol indicating the recipient’s email server. |
| 554, “5.6.0” | The mail message is malformed. Not accepted | Review the structure and content of your email message for any errors or inconsistencies. | Malformed: Incorrectly structured or formatted. |
| 554, “5.6.0” | Message exceeded 50 hops; this may indicate a mail loop | Investigate and resolve any circular forwarding or routing configurations causing the mail loop | Mail loop: A situation where an email is continuously forwarded or routed between multiple recipients. |
| 554, “5.7.0” | Too Many Unauthenticated commands | Review your SMTP authentication settings and ensure proper authentication during the SMTP session. | Unauthenticated commands: SMTP commands are issued without proper authentication. |
| 555, “5.5.2” | Syntax error | Double-check the syntax and format of email addresses, commands, or configuration settings. | Syntax: The rules and structure that govern the correct arrangement of elements in a language or code. |