Espressif ESP32-S2 Programming Manual page 529

Chapter 2. API Reference
MQTT_EVENT_BEFORE_CONNECT
The event occurs before connecting
MQTT_EVENT_DELETED
Notification on delete of one message from the internal outbox, if the message couldn't have been sent
and acknowledged before expiring defined in OUTBOX_EXPIRED_TIMEOUT_MS. (events are not
posted upon deletion of successfully acknowledged messages)
• This event id is posted only if MQTT_REPORT_DELETED_MESSAGES==1
• Additional context: msg_id (id of the deleted message).
enum esp_mqtt_connect_return_code_t
MQTT connection error codes propagated via ERROR event
Values:
MQTT_CONNECTION_ACCEPTED = 0
Connection accepted
MQTT_CONNECTION_REFUSE_PROTOCOL
MQTT connection refused reason: Wrong protocol
MQTT_CONNECTION_REFUSE_ID_REJECTED
MQTT connection refused reason: ID rejected
MQTT_CONNECTION_REFUSE_SERVER_UNAVAILABLE
MQTT connection refused reason: Server unavailable
MQTT_CONNECTION_REFUSE_BAD_USERNAME
MQTT connection refused reason: Wrong user
MQTT_CONNECTION_REFUSE_NOT_AUTHORIZED
MQTT connection refused reason: Wrong username or password
enum esp_mqtt_error_type_t
MQTT connection error codes propagated via ERROR event
Values:
MQTT_ERROR_TYPE_NONE = 0
MQTT_ERROR_TYPE_TCP_TRANSPORT
MQTT_ERROR_TYPE_CONNECTION_REFUSED
enum esp_mqtt_transport_t
Values:
MQTT_TRANSPORT_UNKNOWN = 0x0
MQTT_TRANSPORT_OVER_TCP
MQTT over TCP, using scheme: mqtt
MQTT_TRANSPORT_OVER_SSL
MQTT over SSL, using scheme: mqtts
MQTT_TRANSPORT_OVER_WS
MQTT over Websocket, using scheme:: ws
MQTT_TRANSPORT_OVER_WSS
MQTT over Websocket Secure, using scheme: wss
enum esp_mqtt_protocol_ver_t
MQTT protocol version used for connection
Values:
MQTT_PROTOCOL_UNDEFINED = 0
MQTT_PROTOCOL_V_3_1
MQTT_PROTOCOL_V_3_1_1
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