> ## Documentation Index > Fetch the complete documentation index at: https://apidoc.fax.plus/llms.txt > Use this file to discover all available pages before exploring further. # Fax Fax ## Definition A **Fax** is a document that you want to send to one or more recipients using the Fax.Plus system. Faxes can be in various formats such as PDF, Word, Excel, and other common file types. ## Key Concepts ### Receiving a Fax Fax.Plus can receive faxes on your behalf. In the default setup, faxes are delivered as fully received documents once the transmission is complete, with all pages included in a single file. If you have a use case where you need to start processing pages as soon as they arrive, a fax streaming option (page‑by‑page delivery) is available on demand. Contact us to discuss enabling it for your account. ### Sending a Fax When sending a fax, you typically need to provide: 1. One of your numbers that will be used as a source number 2. The recipient's fax number(s) 3. The document(s) you want to send 4. Any additional options (e.g., cover page, scheduling) Each source number is subject to rate limiting based on your subscription. If you need to send more concurrent faxes than a single number allows, you can load‑balance by providing a comma‑separated list of allowed numbers in the `from` field. The platform will pick one per destination. See the [Send a fax](/api-reference/endpoints/outbox/send-a-fax) endpoint for details. You can upload up to 10 different files per fax, regardless of the number of pages in each file. The maximum total file size is 150 MB. You can include up to 4000 destinations in a single fax request; for larger fan‑out jobs, split your recipients into multiple faxes. If you exceed these limits: * The API returns **413 Payload Too Large** when the total size of all files in a fax exceeds 150 MB. * The API returns **400 Bad Request** with an error code of `too_many_files` when more than 10 files are provided. * The API returns **400 Bad Request** with an error code of `too_many_destinations` when the `to` list contains more than 4000 destinations. ### File Formats Fax.Plus supports a wide range of file formats, including: * Documents: DOC, DOCX, PDF, TXT, RTF * Images: JPG, PNG, TIFF * Spreadsheets: XLS, XLSX Received faxes are available for download through the API in **PDF** and **TIFF** formats (depending on the endpoint and requested content type). ### Cover Pages You can optionally add a fax cover page when sending a fax. A cover page can include: * The sender and recipient names * A subject * A short message or note You can also replace the default Fax.Plus logo in the upper‑right corner of the cover page with your account’s company logo. If no company logo is configured on the account, this flag is silently ignored. To learn how to configure cover sheets and upload your logo in the web app, see the [“How can I add a cover sheet to my fax?” help article](https://help.alohi.com/hc/en-us/articles/10122557790492-How-can-I-add-a-cover-sheet-to-my-fax). ### Fax Status After sending a fax, you can track its status. The status indicates whether the fax was successfully sent, is still in progress, or encountered an error. You can use webhooks to receive real-time updates on fax transmission status. ### Rate limits Faxing is a comparatively slow operation: a single page typically takes around 30–60 seconds to transmit. Because of this, the Fax.Plus API does **not** enforce strict per-endpoint quotas under normal, reasonable usage. However, we do monitor for clearly abusive or unreasonable traffic patterns (for example, polling status every few hundred milliseconds or sending dozens of requests per second). In such cases we may start rate-limiting or throttling requests to protect the platform. **Recommendations:** * Design your integration around the natural pace of faxing (batch sends are fine, but avoid very tight polling loops). * Prefer [webhooks](/concepts/webhook) for status updates instead of frequent polling of fax or outbox endpoints. * Ensure your files are not corrupted or password-protected before sending. * If you expect a large migration or unusually high one-time load, contact us so we can plan capacity together. ### Retries Retries are configured **per submission** when sending a fax. For a detailed view of the retry options available in the request payload, check the [Send a fax](/api-reference/endpoints/outbox/send-a-fax) API reference. * **count**: Number of tries to send the fax. Allowed range: **0–3**. * **delay**: Delay **in seconds** between two retries. Allowed range: **0–180**. By default, when a retry happens the fax is resent starting from page 1. If you enable the **“partially sent retry”** option, retries resume from where the previous attempt stopped instead of starting over. When using partially sent retries, make sure the remote side can accept “chunked” submissions: from their perspective, the retransmitted pages may look like a separate fax. ## Fax Error Statuses Fax.Plus uses a variety of error status codes to indicate the current state of a fax. Here are the possible statuses and their meanings: | Status | Description | | :----------------------------------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | success | The fax was successfully sent. | | partially\_sent | The first pages were transmitted, but the call dropped due to connection issues. | | partially\_received | The first pages were received, but the call dropped due to connection issues. | | in\_progress | The fax is currently being received ; you can retrieve some pages via the API, or wait for the final status. | | insufficient\_credit | Not enough credit to send or receive pages. Add credit or wait for plan reset. | | failed | Generic error. Retry, and contact support if the issue persists. | | failed\_internal\_process\_error | Generic error. Retry, and contact support if the issue persists. | | failed\_user\_busy | The destination was busy. Wait and retry. | | failed\_no\_answer | No answer at the destination. Retry when you know the recipient is available. | | failed\_unallocated\_number | Invalid number. Check the number, including country and area codes. | | failed\_office\_converter\_issue | Failed to convert Microsoft Office document. Recreate and resubmit the file. | | failed\_separate\_file\_pages\_issue | File conversion issue. Check if all pages in the source file(s) are valid. | | failed\_render\_header\_issue | File conversion issue. Check if all pages in the source file(s) are valid. | | failed\_invalid\_number\_format | Invalid number format. Check the number, including country and area codes. | | failed\_mimetype\_not\_supported | Unsupported file type. | | failed\_destination\_not\_supported | Number is a special service number or not supported by your plan. | | failed\_image\_preparation | File conversion issue. Check if all pages in the source file(s) are valid. | | failed\_to\_send | System was busy. Try again later. | | failed\_normal\_temporary\_failure | Temporary network issue. Retry immediately. | | failed\_unknown\_converter\_issue | File conversion failed. Check if the file is password-protected. | | failed\_normal\_clearing | Destination was busy. Wait and retry. | | failed\_convert\_to\_tiff\_issue | File conversion issue. Check if all pages in the source file(s) are valid. | | failed\_fs\_2 / failed\_fs\_3 | The call was established, but there was no fax machine responding to the call. Check the number and retry. | | failed\_fs\_8 / failed\_fs\_9 | Fax protocol incompatibility between the two endpoints. Check that the recipient is using a standard fax machine or service, and retry. | | failed\_fs\_31 / failed\_fs\_32 | The remote fax stopped responding during the call (for example, due to line issues or device lockups). Retry later; if the issue persists, contact the recipient. | | failed\_fs\_35 / failed\_fs\_39 | The remote fax disconnected unexpectedly or hung up before the transmission completed. Retry later; if the issue persists, contact the recipient. | | failed\_fs\_48 | Remote fax machine disconnected after the same message was sent multiple times unsuccessfully. Some pages may have been transmitted. | | failed\_fs\_49 | Remote fax machine disconnected unexpectedly. Some pages may have been transmitted. | | failed\_fs\_\* | Fax communication error (for example, line quality, timeouts, or other low-level fax protocol issues). Retry later; if the issue persists, contact support and include the fax ID. | ## Best Practices 1. **File Preparation**: Ensure your files are not corrupted or password-protected before sending. 2. **Number Verification**: Double-check fax numbers, including country and area codes. 3. **Retry Strategy**: For temporary failures, implement a retry mechanism with appropriate intervals. ## Schema