> For the complete documentation index, see [llms.txt](https://docs.sendman.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.sendman.com/other/troubleshooting/smtp-reply-codes.md). # SMTP Reply Codes Replies your clients may receive from Sendman, their causes and how to fix them The following replies are generated by Sendman itself. All other replies come from your [SMTP backend](/admin-portal/settings/smtp-backend.md) and are passed on to the client unchanged. | Reply | Cause | Solution | | ------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | *Connection closed without a reply* | The client's address is not on the IP allow list. | Add the client's address to the [IP Allow List](/admin-portal/sendman/ip-allow-list.md). | | `421 4.3.0 Service not available, closing transmission channel` | No SMTP backend is configured, or the configuration is temporarily unavailable. | Configure the [SMTP backend](/admin-portal/settings/smtp-backend.md). If it is configured, the client should retry later; contact [support](broken://pages/LMXInC2sPA0JU8B7xRXT) if the problem persists. | | `421 4.4.2 Timeout, closing transmission channel` | The client sent no command for 2 minutes, or the connection lasted longer than 1 hour. | Check the client's network connection. Clients should open a new connection for later messages. | | `500 5.5.2 Line too long` | The client sent a command line longer than allowed. | Check the client's configuration; this usually indicates a faulty client. | | `535 Authentication failed. Please try again.` | Username or password are wrong, or the Entra ID sign-in was refused. | Check the [SMTP credential](/admin-portal/sendman/smtp-credentials.md) or the Entra ID account. For Entra ID, see [Troubleshooting](/other/troubleshooting.md#entra-id-sign-in-fails-although-the-password-is-correct). | | `538 5.7.11 Encryption required for requested authentication mechanism` | The client tried to sign in on port 587 or 465 without TLS. | Enable STARTTLS or SSL/TLS on the client, or send on port 25 through a [network connection](/admin-portal/settings/network-connections.md). | | `550 5.7.60 Sender address rejected: not authorized` | No applicable [authorization policy](/admin-portal/sendman/authorization-policies.md) allows the sender address (`MAIL FROM`). | Check the **Allowed senders** of the policies that apply to the client, and their priorities. | | `550 5.7.1 Recipient not authorized, your account is not permitted to send to this address` | The policy that allows the sender does not allow this recipient (`RCPT TO`). | Check the **Allowed receivers** of the policy, or tick **Allow any recipient**. | | `552 5.3.4 Message size exceeds fixed maximum message size` | The message is larger than 25 MB, or larger than the limit of your SMTP backend. | Reduce the size of the message, e.g. by lowering the scan resolution. | --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://docs.sendman.com/other/troubleshooting/smtp-reply-codes.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.