| Code | Error Message | Meaning | Solution | Glossary |
| 400 | from parameter is missing | The sender's email address is not provided | Include the sender's email in the 'from' parameter | 'from' parameter: Sender's email address |
| 400 | to parameter is missing | The receiver's email address is not provided | Include the receiver's email in the 'to' parameter | 'to' parameter: Receiver's email address |
| 400 | message parameter is missing | The email body is not provided | Include the email body in the 'message' parameter | 'message' parameter: Email body |
| 400 | ‘message’ parameter is not a file | The 'message' parameter is expected to be a file but isn't | Provide the message as a file | 'message' parameter: Email body, in this case expected as a file |
| 400 | Need at least one of ‘text’ or ‘html’ parameters specified | Neither plain text nor HTML versions of the email are provided | Include at least one of the 'text' or 'html' parameters | 'text'/'html' parameters: Plain text/HTML versions of the email |
| 400 | Only one parameters ‘html’ or ‘template’ is allowed | Both 'html' and 'template' parameters are provided, but only one is allowed | Provide either the 'html' or 'template' parameter, not both | 'html'/'template' parameters: HTML version of the email / Template for the email |
| 400 | Send options (parameters starting with o:, h:, or v:) are limited to 16 kB total | The send options provided exceed the 16 kB limit | Reduce the size of send options to less than 16 kB | 'o:', 'h:', 'v:' parameters: Send options for the email |
| 400 | Too many recipients; max is 1000 | The email is addressed to more than 1000 recipients | Limit the number of recipients to 1000 or less | Recipients: The individuals or groups receiving the email |
| 400 | Header name must be pure ASCII: <header-name> | The header name contains non-ASCII characters | Use only ASCII characters in the header name | ASCII: American Standard Code for Information Interchange, a character encoding standard |
| 400 | <invalid-date> is not an RFC-2822 compliant date | The provided date is not compliant with RFC-2822 | Ensure the date is formatted according to RFC-2822 standards | RFC-2822: Internet Message Format specification |
| 400 | to parameter is not a valid address. please check documentation | The receiver's email address is invalid | Check and correct the format of the receiver's email address | 'to' parameter: Receiver's email address |
| 400 | Invalid request content type. Expecting ‘multipart/form-data’ but got ‘application/x-www-form-urlencoded’ | The content type of the request is not what is expected | Change the request content type to 'multipart/form-data' | 'multipart/form-data': A type of HTTP request commonly used for file uploads |
| 400 | malformed multipart/form-data request | The 'multipart/form-data' request is formatted incorrectly | Correct the format of the 'multipart/form-data' request | 'multipart/form-data': A type of HTTP request commonly used for file uploads |
| 400 | unable to parse request: invalid URL escape | The URL in the request contains invalid escape characters | Correct the escape characters in the URL | URL escape characters: Characters used in a URL to represent certain other characters |
| 400 | unable to parse request: unexpected EOF | The request ends unexpectedly | Check the request for missing information or incorrect formatting | EOF: End of File, in this case referring to the end of the request |
| 400 | <time> invalid 24 hour time(value of ‘o:time-zone-localize’) | The time provided is not in a valid 24-hour format | Ensure the time is in a valid 24-hour format | 'o:time-zone-localize': A parameter for setting the time zone |
| 400 | <time> is not a valid time to send (value of ‘o:time-zone-localize’) | The specified time is not a valid time to send the email | Choose a different, valid time to send the email | 'o:time-zone-localize': A parameter for setting the time zone |
| 400 | invalid delivery time format | The format of the delivery time is invalid | Correct the format of the delivery time | Delivery time: The time when the email should be delivered |
| 400 | to parameter is not a valid address. please check documentation | The receiver's email address is invalid | Check and correct the format of the receiver's email address | 'to' parameter: Receiver's email address |
| 400 | from parameter is not a valid address. please check documentation | The sender's email address is invalid | Check and correct the format of the sender's email address | 'from' parameter: Sender's email address |
| 400 | cc parameter is not a valid address. please check documentation | The CC email address is invalid | Check and correct the format of the CC email address | 'cc' parameter: Carbon copy email address |
| 400 | bcc parameter is not a valid address. please check documentation | The BCC email address is invalid | Check and correct the format of the BCC email address | 'bcc' parameter: Blind carbon copy email address |
| 400 | ‘recipient-variables’ parameter is not a valid JSON | The 'recipient-variables' parameter is not correctly formatted as JSON | Correct the format of the 'recipient-variables' parameter | 'recipient-variables' parameter: Variables specific to each recipient of the email |
| 400 | Domain example.com is not allowed to send: Sandbox subdomains are for test purposes only. Please add your own domain or add the address to authorized recipients in Account Settings. | The sender's domain is a sandbox subdomain, which are for testing only | Use a non-sandbox domain, or add the recipient to the list of authorized recipients | Sandbox domain: A domain for testing purposes |
| 400 | Domain example.com is not allowed to send: Free accounts are for test purposes only. Please upgrade or add the address to authorized recipients in Account Settings. | The sender's account is a free account, which is for testing only | Upgrade to a paid account, or add the recipient to the list of authorized recipients | Free account: An account that doesn't require payment |
| 400 | Domain example.com is not allowed to send: The domain is unverified and requires DNS configuration. Log in to your control panel to view required DNS records. | The sender's domain is unverified and needs DNS configuration | Verify the domain and configure the DNS as needed | DNS: Domain Name System, a system for translating domain names to IP addresses |
| 400 | Domain example.com is not allowed to send: Please activate your Mailgun account. Check your inbox or log in to your control panel to resend the activation email. | The sender's Mailgun account is not activated | Activate the Mailgun account by following the instructions in the activation email | Mailgun: A service for sending, receiving and tracking emails |
| 400 | is not a valid secondary dkim domain name | The provided secondary DKIM domain name is invalid | Check and correct the format of the secondary DKIM domain name | DKIM: DomainKeys Identified Mail, a method for verifying the authenticity of email senders |
| 401 | Forbidden | The user does not have the necessary permissions for the request | Ensure the user has the correct permissions for the request | Forbidden: A status code indicating the user doesn't have necessary permissions |
| 404 | Domain not found: example.com | The specified domain does not exist | Use a valid, existing domain | Domain: The part of an email address after the @ symbol |
| 403 | Domain example is not allowed to send large batches yet | The specified domain is not authorized to send large batches of emails | Request permission to send large batches, or send smaller batches | Batch: A group of emails sent as part of a single operation |
| 403 | Rejected: IP <id-address> can’t be used to send the message | The IP address of the sender is not allowed to send the message | Use a different, authorized IP address to send the message | IP address: A unique identifier for devices on a network |
| 413 | request size exceeds 52.4MiB limit | The size of the request exceeds the limit of 52.4MiB | Reduce the size of the request to under 52.4MiB | MiB: Mebibyte, a unit of digital storage |
| 429 | Domain example.com is not allowed to send: request limit exceeded, try again after Mon, 20 Dec 2021 20:33:21 UTC | The sender's domain has exceeded the limit for sending requests | Wait until the specified time, then try sending the request again | Request limit: The maximum number of requests that can be sent in a given time period |
| 429 | Domain example.com is not allowed to send: bytes limit exceeded, try again after Mon, 20 Dec 2021 20:33:21 UTC | The sender's domain has exceeded the limit for sending bytes | Wait until the specified time, then try sending the request again | Bytes limit: The maximum amount of data that can be sent in a given time period |
| 429 | Domain example.com is not allowed to send: recipient limit exceeded, try again after Mon, 20 Dec 2021 20:33:21 UTC | The sender's domain has exceeded the limit for number of recipients | Wait until the specified time, then try sending the request again | Recipient limit: The maximum number of recipients that can be addressed in a given time period |
| 500 | Internal Server Error | There is a problem with the server | Wait and try the request again later, or contact the server administrator | Internal Server Error: A general-purpose status code indicating a problem with the server |
