Fires when user sends message headers.

Private Sub object_HeadersReceived(User, Headers)

The HeadersReceived Event syntax has these parts:

This event is fired immediately after user sends mail headers, but before any body is received, and before headers are written to the local file. At this point you can access Headers collection and add, change, remove headers from it as you wish.

You cannot deny accepting further message because SMTP protocol does not allow it.