> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sendpost.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom SMTP Event Types

> Understand Custom SMTP (CS) event types and how they map to the sending lifecycle.

Custom SMTP events use the **CS** prefix and specific event type IDs in SendPost, so you can distinguish them from native SendPost and other TPSP events.

***

## Custom SMTP Event Types

| Event Type    | ID | Trigger                                                                                                                                       |
| ------------- | -- | --------------------------------------------------------------------------------------------------------------------------------------------- |
| CSProcessed   | 58 | SendPost accepted the message and queued it for relay to your SMTP server                                                                     |
| CSSent        | 59 | Your MTA returned 2xx on relay (message accepted by your server for delivery)                                                                 |
| CSDelivered   | 60 | Agent reports MTA `status=sent` (delivered to recipient's mail server)                                                                        |
| CSDeferred    | 61 | Agent may send `deferred`; **accepted by API but not stored** (not shown in UI)                                                               |
| CSDropped     | 62 | Relay to your SMTP failed after all retries; or non-retryable pre-send failure (e.g. no verified return path domain, suppression, validation) |
| CSHardBounced | 63 | Agent `bounced`/`soft_bounced` classified as hard (by SMTP description); or bounce processor (async DSN)                                      |
| CSSoftBounced | 64 | Agent `bounced`/`soft_bounced` classified as soft (by SMTP description); or bounce processor                                                  |
| CSOpen        | 65 | Recipient loaded the tracking pixel                                                                                                           |
| CSClick       | 66 | Recipient clicked a tracked link                                                                                                              |
| CSSpamReport  | 67 | FBL complaint (out of scope for initial release)                                                                                              |
| CSUnsubscribe | 68 | Recipient used the unsubscribe link/header                                                                                                    |

***

## Event Flow

<AccordionGroup>
  <Accordion title="CSSent vs CSDelivered" icon="circle-info">
    **CSSent** = Your MTA accepted the message from SendPost (synchronous SMTP 2xx response).
    **CSDelivered** = Your MTA successfully delivered to the recipient's mail server (reported by the event agent from your MTA logs).
    This mirrors SendPost's native model: Sent = accepted by sending infrastructure; Delivered = accepted by recipient's mail server.
  </Accordion>

  <Accordion title="CSDropped" icon="circle-xmark">
    Emitted when SendPost could not relay to your SMTP server after all retries (connection failure, timeout, or 4xx/5xx from your server). This is a relay failure, not a recipient bounce. CSHardBounced is only from the agent (MTA status=bounced) or the bounce processor (async DSN).
  </Accordion>

  <Accordion title="Bounces: Agent vs Bounce Processor" icon="rotate-left">
    **Agent**: Sends `bounced` or `soft_bounced`; SendPost classifies each to CSHardBounced or CSSoftBounced using the SMTP description (keyword-based). **Bounce processor**: Receives async DSNs to `sp-bounce.{domain}` and emits CSHardBounced or CSSoftBounced when the original message was sent via Custom SMTP.
  </Accordion>

  <Accordion title="CSDeferred not in UI" icon="circle-info">
    The agent may send `deferred` events. SendPost accepts them but **does not store** deferred events (by design). CSDeferred (61) will not appear in the Events page or analytics.
  </Accordion>
</AccordionGroup>

***

## Event Sources Summary

| Source                                  | Events                                                                       |
| --------------------------------------- | ---------------------------------------------------------------------------- |
| SendPost queue                          | CSProcessed                                                                  |
| Synchronous SMTP response from your MTA | CSSent (2xx), CSDropped (after retries exhausted)                            |
| Event reporting agent (MTA logs)        | CSDelivered, CSHardBounced, CSSoftBounced (deferred accepted but not stored) |
| SendPost tracking                       | CSOpen, CSClick, CSUnsubscribe                                               |
| Bounce processor                        | CSHardBounced, CSSoftBounced                                                 |

***

## Next Steps

* [Event reporting agent](/guides/custom-smtp/event-agent) for CSDelivered, CSDeferred, CSHardBounced from your MTA
* [Analytics](/guides/custom-smtp/analytics) to view event statistics
* [Troubleshooting](/guides/custom-smtp/troubleshooting) event processing