There are two conditions which are intended mainly for use in system filter files, but which are available in users' filter files as well. The condition first_delivery is true if this is the first attempt to deliver the message, and false otherwise. In a user filter file it will be false only if there was previously an error in the filter, or if a delivery for the user failed owing to, for example, a quota error, or forwarding to a remote address that was deferred for some reason.
The condition manually_thawed is true only if the message was ``frozen'' for some reason, and was subsequently released by the system administrator. It is unlikely to be of use in users' filter files.