> ## Documentation Index > Fetch the complete documentation index at: https://ahasend.com/docs/llms.txt > Use this file to discover all available pages before exploring further. > ## Agent Instructions > AhaSend stores all message data in the EU. See /facts. > The API base URL is https://api.ahasend.com. Send the API key in the Authorization: Bearer header. See /api-reference/authentication. > Use send.ahasend.com for SMTP. Ports 25, 587 and 2525 use STARTTLS. Port 465 is not supported. See /smtp. > For send-only integrations, create an API Key v2 under Credentials → Add and scope it to messages:send:{your-domain}. See /send-api/credentials and /api-reference/scopes. # SMTP Error Codes and Delivery Fixes > Read SMTP error codes from AhaSend and recipient servers, including 550 5.1.1, authentication failures, temporary deferrals and sender-domain problems. AhaSend SMTP errors tell you whether a connection, submission or recipient delivery failed; this reference helps you read the full reply and choose the next step. ## Where Did the Error Occur? An error during your application's connection to AhaSend is a submission problem. A reply recorded later in a delivery attempt came from the destination mail system. Fix the component that produced the reply: changing your SMTP password will not repair a nonexistent recipient mailbox. The three-digit SMTP reply gives a broad result. An enhanced code such as `5.1.1` adds detail. A `4xx` reply is generally temporary; a `5xx` reply is generally permanent for that attempt. Read the accompanying text and sender or recipient address. The [SMTP standard](https://www.rfc-editor.org/rfc/rfc5321.html) and [enhanced status code registry](https://www.iana.org/assignments/smtp-enhanced-status-codes/smtp-enhanced-status-codes.xhtml) define these classes. ## What Do Common Codes Mean? These are common SMTP meanings, not a promise that every server emits identical text. | Reply or enhanced code | Meaning | Next step | | - | - | - | | `220` | Server is ready | Continue the SMTP exchange | | `235` | Authentication succeeded | Submit the message | | `250` after message data | That server accepted the message | Track delivery separately; do not immediately resend | | `421` | Service unavailable or closing the connection | Retry later with a bounded delay after checking service status | | `450`, `451`, `452` | Temporary mailbox, processing or resource problem | Read the text and let the delivery queue retry | | `451 4.3.0 Unsubscribe link is not available` | AhaSend could not make the [unsubscribe link](/docs/campaigns/unsubscribe.md) that the message needs | Send the message again later; SMTP clients do this automatically | | `530` | Authentication or a required security step is missing | Enable STARTTLS, then authenticate as the reply requires | | `535` / `5.7.8` | Authentication credentials are invalid | Check the SMTP username and password, not an API key | | `550 5.1.1` | Destination mailbox address does not exist | Correct the address; do not repeatedly retry it | | `5.2.2` | Mailbox is full | Follow the accompanying 4xx or 5xx reply and provider text | | `552` / `5.3.4` | Message exceeds a size limit (AhaSend: 20 MiB per message) | Reduce content or attachments; base64 adds about one third to attachment size | | `552` / `5.6.0` | AhaSend rejected a malformed message, for example one with bare line feeds instead of CRLF line endings or no `From` header | Fix the structure or headers named in the reply, then resend | | `5.7.1` | Delivery refused for a security or policy reason | Inspect authentication, content, sender reputation and the server's text | | `550 5.7.1 Sending from domain '' is paused` | AhaSend paused sending from that domain | See [paused domains](/docs/domains.md#why-is-sending-paused-on-my-domain) | | `5.7.26` | Multiple authentication checks failed | Check SPF, DKIM and DMARC alignment | ## Why Does AhaSend Return a 556 Domain Error? An AhaSend submission reply such as `556 Invalid account or domain` or a reply saying the From domain is not allowed means the account, sender domain or credential's domain access needs checking. Confirm that the From address uses a verified domain allowed by that credential. For a newly added or verified domain, use the refresh timing on [domain setup](/docs/domains.md#troubleshooting). Do not infer that every `556` from every recipient server has this AhaSend-specific meaning. Keep the exact reply when reporting a problem. ## Why Does Port 465 Fail? Port 465 uses implicit TLS: set your client to SSL/TLS, not STARTTLS. A client set to STARTTLS on port 465 waits for a greeting, and the server closes the connection after 30 seconds. See the settings for common libraries in [SMTP settings](/docs/smtp.md#does-ahasend-support-port-465). Do not disable certificate validation to get past a connection error. If your host blocks outbound SMTP, check its network policy or use the [HTTPS API](/docs/send-api/send-email.md). ## Should I Retry a Failed Send? Let AhaSend handle recipient delivery attempts after it accepts a message. A temporary delivery event is not a request for your app to send another copy. If the connection drops before you receive the final submission reply, the outcome can be uncertain; check the message log before retrying. SMTP has no equivalent of the REST API's [idempotency key](/docs/api-reference/idempotency.md). ## Related Guides * [SMTP credentials](/docs/smtp/credentials.md), [SMTP hosts, ports and limits](/docs/smtp.md), [domain setup](/docs/domains.md) and [email authentication](/docs/email-authentication.md). * [Bounces and suppression durations](/docs/bounces.md), [message logs API](/docs/api-reference/messages/get-messages.md), [SMTP CLI](/docs/smtp/cli.md) and [Node.js SMTP](/docs/smtp/nodejs.md). * [REST API errors](/docs/api-reference/errors.md), [Node.js SDK](/docs/guides/nodejs-sdk.md) and [troubleshooting](/docs/troubleshooting.md). This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.