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

CSSent vs CSDelivered

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.

CSDropped

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).

Bounces: Agent vs Bounce Processor

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.

CSDeferred not in UI

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.

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

Documentation

Guides

SDK Quickstarts

Common Use Cases

Custom SMTP Event Types