John,

SpamFilter is a multi-threaded application, so each task can be performed by a separate thread and ran concurrently with other tasks. Each ID that appears in the logs is a "thread ID" and shows what a particular thread is doing. Each email that arrives is processed by a single thread. So if you follow the logs and select all the lines with the same thread ID, from the "Connection" to the "Disconnect" event, you will have the "history" of an email from begin to finish. The messages that are forwarded to your mail server are handled by a separate thread, so the IDs for the forwarding will be different.

Windows re-used thread IDs when a thread terminates, so the same number may appear again in the logs.