✉️ Email Delivery
Documentation, reference material and tutorials for CRM-NG SMTP Email: setup guides for every mail service, troubleshooting, and one article per error message.
Error library
60 articlesOne article per error message, with what it means and how to fix it.
- Error: 400 invalid_request (Microsoft 365)Microsoft refused the token request because a value in it is missing or wrong, usually the Tenant ID or the Application (Client) ID.
- Error: 401 Unauthorized or ForbiddenAn API mailer rejected CRM-NG SMTP's credentials (401) or refused the action (403). Check the key, its permissions and the region.
- Error: 404 Not FoundThe mail service could not find what CRM-NG SMTP asked for, usually a domain, region or mailbox. Check those settings against your account.
- Error: 421 Connection timeout: network issuesThe mail server is temporarily refusing service and closed the connection. Wait and retry; if it keeps happening, slow down or check your server's IP.
- Error: 442 Connection dropped during transmissionThe connection to the mail server broke while the message was being sent. Usually a network or server hiccup; retry, then look for a pattern.
- Error: 447 Message or recipient limit reachedThe mail server has temporarily stopped accepting mail from you because you passed its message or recipient limit. Slow down and retry later.
- Error: 450 Requested mail action not taken: mailbox unavailableThe server could not handle the mailbox right now and asked you to try later. Usually temporary; persistent cases point to DNS or policy.
- Error: 451 Temporary local problem or timeoutThe mail server hit a temporary problem of its own while handling your message. Retry later; if it persists, check DNS and reputation.
- Error: 452 4.5.3 Domain policy size per transaction exceededOne email was addressed to more recipients than the server allows per message. Split it into smaller sends or trim CC and BCC lists.
- Error: 521 Server does not accept mailThe server you reached does not take email at all, either for the recipient's domain or for sending. Check the address and your SMTP host.
- Error: 534: application-specific password requiredThe SMTP server will not accept your account's normal password. Create an app password, or switch to a mailer that uses OAuth.
- Error: 535 Authentication failed: incorrect login credentialsThe SMTP server refused the username and password CRM-NG SMTP sent. Check the login, use an app password where needed, and confirm SMTP login is allowed.
- Error: 550 Host unknown: incorrect DNS settingsA domain in the message, usually the recipient's, has no mail records in DNS. Check the spelling of the address and the domain's MX records.
- Error: 550 Requested action not taken: address does not exist or was rejectedThe server permanently refused the message for this recipient: the mailbox does not exist, relaying was denied, or a policy blocked it.
- Error: 553 Sender address rejected: invalid sender email addressThe SMTP server will not send from your From address. Use the mailbox you log in with, or an alias it is allowed to use, and force it.
- Error: 554 Transaction failedThe server permanently refused the message, usually for spam, blocklist or authentication reasons. Read the detail, fix the cause, then resend.
- Error: AADSTS50194: single-tenant application (Microsoft 365)The Tenant ID field holds a shared value such as "common" instead of your own tenant. Enter your Directory (tenant) ID.
- Error: Access denied (Elastic Email)Elastic Email accepted the API key but will not let it send. Fix the key's permissions or IP limit, or the account's status.
- Error: Account blocked: malicious behaviourThe mail service has suspended your account after its abuse checks flagged activity. Find the cause, secure the site, then ask the provider to review.
- Error: Account not verified (SendLayer)SendLayer will not send until your account is verified. Confirm your email address and finish any account review SendLayer asks for.
- Error: API key expired (Elastic Email)The Elastic Email API key saved in CRM-NG SMTP has passed its expiry date. Create a new key and replace the old one.
- Error: Authenticated user not authorized to send (SendGrid)SendGrid knows your API key but will not let it send mail. The key lacks Mail Send permission or the account is restricted.
- Error: Authorization grant invalid or expired (SendGrid)SendGrid does not accept the API key saved in the plugin. It is mistyped, deleted or from another account. Create a new key and save it.
- Error: Channel not found (SMTP.com)The Sender Name saved in the plugin does not match a sender (channel) in your SMTP.com account. Copy the exact name from SMTP.com.
- Error: Connection refusedThe mail server's address answered but refused the connection on that port. Check the port and host, then ask your host about firewalls.
- Error: Connection timed out (code 110)Your server waited for the mail server or API and got no answer. Usually a blocked port or firewall; sometimes a slow or unreachable service.
- Error: Could not connect to SMTP hostCRM-NG SMTP could not open a connection to the mail server. Check host, port and encryption, then ask your host whether the port is blocked.
- Error: cURL errorYour server could not complete the HTTPS request to an API mailer. The number after "cURL error" tells you whether it is DNS, connection or certificates.
- Error: cURL error 77: certificate problem (Gmail)Your web server cannot read its list of trusted certificates, so it cannot open a secure connection to Google. Your host has to fix it.
- Error: Domain is not activated (SendLayer)SendLayer has not confirmed the DNS records for your sending domain. Add the records it lists, verify them, and send from that domain.
- Error: Domain not verified (MailerSend)MailerSend will not send from a domain it has not verified. Add the domain's DNS records, verify them, and send from that domain.
- Error: Email address in suppression listYour mail service refuses to send to a recipient it has blocked after an earlier bounce, complaint or unsubscribe. Check why before removing it.
- Error: Email address is not verified (Amazon SES)Amazon SES refused the email because the sender, or in the sandbox the recipient, is not a verified identity in the region you selected.
- Error: Email sending limit reachedYour mail service or mailbox has hit its daily, monthly or per-second quota. Wait, raise the limit, or spread sending out with rate limiting.
- Error: ENDPOINT_PERMISSION_DENIED (SMTP2GO)The SMTP2GO API key is valid but not allowed to use the email-sending endpoint. Give the key sending permission and test again.
- Error: ErrorSendAsDenied (Microsoft 365)Microsoft 365 will not let the mailbox send with the From address you chose. Use a real mailbox's primary address as the From Email.
- Error: From address does not match a verified Sender Identity (SendGrid)SendGrid refuses mail from an address it has not verified. Verify the sender or authenticate the domain, then use that address as the From Email.
- Error: Gmail could not authenticateGmail's SMTP server refused the username and password saved in the Other SMTP mailer. Use an app password, or switch to the Gmail mailer.
- Error: Hostname could not be resolvedYour web server could not turn the mail server's name into an address. Check the host name for typos, then ask your host about DNS.
- Error: Invalid API keyThe mail service did not recognise the API key CRM-NG SMTP sent. Copy a fresh key from the provider and paste it into the mailer settings.
- Error: Invalid signature or credentials (Amazon SES)AWS could not match the request to your access key. The Access Key ID or Secret Access Key is wrong, inactive, or the server clock is off.
- Error: invalid_grant: bad request (Gmail)Google rejected the Gmail token request as malformed. Usually the server clock is wrong or the saved token belongs to another OAuth client.
- Error: invalid_grant: token expired or revoked (Gmail)Google no longer accepts the saved Gmail connection. Find out why the token stopped working and reconnect the Google account.
- Error: InvalidAuthenticationToken: token expired (Microsoft 365)Microsoft Graph rejected the access token the plugin sent. Usually the server clock is off or a cached token is stale.
- Error: InvalidInternetMessageHeader (Microsoft 365)Microsoft Graph refused a custom header added to the email. Find the plugin adding it, then fix or remove that header.
- Error: Mail service not enabled (Gmail)The connected Google account has no Gmail mailbox, so Google cannot send for it. Turn Gmail on for the account or connect one that has it.
- Error: MailboxNotEnabledForRESTAPI (Microsoft 365)The From Email mailbox is not an active Exchange Online mailbox, so Microsoft Graph cannot send from it. License it or pick another mailbox.
- Error: MessageRejected (Amazon SES)Amazon SES accepted the request but refused to send this message. The text after MessageRejected says why: identity, sandbox, suspension or content.
- Error: One-Click Setup request refused by the relayThe CRM-NG sign-in relay answered, but refused the request or returned a result your site could not read.
- Error: One-Click Setup sign-in cancelledThe sign-in at Google or Microsoft was cancelled or the permission to send email was not granted, so no connection was stored.
- Error: One-Click Setup sign-in could not be renewedThe provider would not renew the stored sign-in, or the stored sign-in has no usable token, so sending stops until you sign in again.
- Error: One-Click Setup sign-in relay unreachableYour site could not reach the CRM-NG sign-in relay, or the relay is not configured on this site, so the sign-in or the renewal could not run.
- Error: Permission denied: SMTP account not activated (Brevo)Brevo has not switched on transactional sending for your account yet. Complete the account checks or ask Brevo to activate it.
- Error: Sender domain not validated (Elastic Email)Elastic Email will not send from a domain it has not verified. Add the DNS records it asks for, verify the domain, then test again.
- Error: SSL certificate verify failedYour server could not trust the mail server's certificate. Use the host name on the certificate and ask your host to update its CA bundle.
- Error: Trial accounts can only send to the admin address (MailerSend)Your MailerSend account is still in its trial state, so it may only send to the account owner's address. Verify a domain and finish approval.
- Error: Unauthenticated (MailerSend)MailerSend refused the request because the API key CRM-NG SMTP sent is missing, wrong or deleted. Create a fresh key and paste it in.
- Error: unauthorized_client (Gmail)Google refuses to let this OAuth client use the saved Gmail token. The client changed, was disabled, or is the wrong type.
- Error: Unauthorized: key not found (Brevo)Brevo cannot find the API key the plugin sent. It was deleted, mistyped, or is an SMTP key rather than a v3 API key.
- Error: Unauthorized: not verified or IP not authorized (Brevo)Brevo refused the API call because your server's IP address is not on the account's allowed list, or the account is not verified yet.