# Customer.io Send Messages API

**Canonical:** https://apis.io/apis/customer-io/customer-io-send-messages-api/  
**Provider:** Customer.io — https://apis.io/providers/customer-io/  
**Base URL:** https://track.customer.io  
**Documentation:** https://docs.customer.io/integrations/api/track/

Customer.io Send Messages API is one of 42 APIs that [Customer.io](https://apis.io/providers/customer-io/) publishes on the [APIs.io](https://apis.io/) network, described by a machine-readable OpenAPI specification. Tagged areas include Send Messages. The published artifact set on APIs.io includes an OpenAPI specification and API documentation.

Use these endpoints to send broadcasts or transactional messages to your audience. They use the same authentication method, but review the limits for both types of workflows to make sure you'll successfully trigger your messages. You can't trigger campaigns through our APIs. ## API triggered broadcast limits While [API triggered broadcasts](#operation/triggerBroadcast) are limited to 1 request every 10 seconds, they're also limited by the number of triggers you can have queued at a time, as shown in the table below. | Data Type | Limit | Description | | -- | -- | -- | | Trigger Payload | 25MB | Max length of the entire Trigger call, larger calls are typically to support `per_user_data` | | Trigger Data | 50000 bytes | Max length of the data section in the Trigger call | | Recipient List<sup>1</sup> | 10000 recipients | Max number of ids or emails included in the Trigger call | | Custom Per User Data<sup>1</sup> | 10000 entries | Max number of entries in `per_user_data` | | Custom Per User Data<sup>1</sup> | 2MB | Max length per user in the file referenced by `data_file_url`. | | Custom Per User Data<sup>1</sup> | 10GB | Max length of the entire size of the file referenced by `data_file_url` | | Trigger Queue | 5 triggers | Max number of triggers waiting to be processed consecutively | <sup>1</sup>For larger data sets, use `data_file_url` to supply a link to a file that contains your merge data. Attempting to send too much data in a single API call will fail. ## Transactional Message Limits The `/send-*` endpoints are limited to 100 requests per second. | Data Type | Limit | Description | | -- | -- | -- | | Payload | 1MB | Max length of the payload, excepting attachments | | Attachments (email) | 2MB | Maximum size of attachments | | Recipients | 15 | Total number of recipients across the `to`, `cc`, and `bcc` fields. |

## Machine-readable artifacts (3)

- **OpenAPI** — https://raw.githubusercontent.com/api-evangelist/customer-io/refs/heads/main/openapi/customer-io-send-messages-api-openapi.yml
- **Documentation** — https://docs.customer.io/integrations/api/app/
- **Documentation** — https://docs.customer.io/integrations/data-in/connections/http-api/

## Other Customer.io APIs (12)

- [Customer.io Track API](https://apis.io/apis/customer-io/track-api/)
- [Customer.io Activities API](https://apis.io/apis/customer-io/customer-io-activities-api/)
- [Customer.io Alias API](https://apis.io/apis/customer-io/customer-io-alias-api/)
- [Customer.io Batch API](https://apis.io/apis/customer-io/customer-io-batch-api/)
- [Customer.io Broadcasts API](https://apis.io/apis/customer-io/customer-io-broadcasts-api/)
- [Customer.io Campaigns API](https://apis.io/apis/customer-io/customer-io-campaigns-api/)
- [Customer.io Collections API](https://apis.io/apis/customer-io/customer-io-collections-api/)
- [Customer.io Customers API](https://apis.io/apis/customer-io/customer-io-customers-api/)
- [Customer.io Exports API](https://apis.io/apis/customer-io/customer-io-exports-api/)
- [Customer.io Group API](https://apis.io/apis/customer-io/customer-io-group-api/)
- [Customer.io Identify API](https://apis.io/apis/customer-io/customer-io-identify-api/)
- [Customer.io Messages API](https://apis.io/apis/customer-io/customer-io-messages-api/)

## Tags

Send Messages

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Profiled by [API Evangelist](https://apievangelist.com) and published on [APIs.io](https://apis.io/apis/customer-io/customer-io-send-messages-api/). The API's provider profile, Kin Score and agent-readiness rating are at https://apis.io/providers/customer-io/.