[This is preliminary documentation and subject to change.]
An SMTP service event is the occurrence of some activity within the service, such as the transmission or arrival of an SMTP protocol command, or the submission of a message into the SMTP service transport component. When a particular event occurs, the SMTP service uses an event dispatcher to notify registered event sinks of the event. When notifying event sinks, the service passes information to the sink in the form of COM object references.
SMTP server events are broadly broken down into two categories:
Protocol Events
These events occur when SMTP protocol commands are either received or transmitted over the network. These events occur when:
A client SMTP service or messaging user agent (MUA) transmits messages for delivery to the local service using the SMTP protocol.
The SMTP service relays messages to other SMTP services.
Transport Events
These events occur when a message has been received by the service and that message passes through the SMTP core transport. During the passage through the transport, the message is categorized (examined and placed into categories) and then either delivered to a local storage location or relayed to another destination if it is not local.