When in the course of delivery a message is processed by a filter file, what happens next, that is, after the whole filter file has been processed, depends on whether the filter has set up any significant deliveries or not. If there is at least one significant delivery, the filter is considered to have handled the entire delivery arrangements for the current address, and no further processing of the address takes place. If, however, no significant deliveries have been set up, Exim continues processing the current address as if there were no filter file, and typically sets up a delivery of a copy of the message into a local mailbox. In particular, this happens in the special case of a filter file containing only comments.
The delivery commands deliver, save, and pipe are by default significant. However, if such a command is preceded by the word unseen, its delivery is not considered to be significant. In contrast, other commands such as mail and vacation do not count as significant deliveries unless preceded by the word seen.