See qmail(7), qmail-local(8), qmail-command(8), envelopes(5), and dot-qmail(5). Briefly, qmail having resolved the delivery address delivers it via the .qmail file that most completely matches the address. This file may be a link to another file, as is the case in ezmlm lists. qmail then delivers the message according to successive lines in this file forwarding it to an address, storing it, or piping it to a program. In the latter case, the program is expected to exit 0 leading delivery to proceed to the next line in the .qmail file, or 99 leading to success without delivery to succeeding lines. An exit code of 100 is a permanent error leading to an error message to the SENDER. An exit code of 111 is used for temporary errors, leading to re-delivery until successful or until the queue lifetime of the message has been exceeded.
Delivery granularity is the .qmail file and re-deliveries start at the top. Thus, if the message fails temporarily at a later line, the delivery according to an earlier line will be repeated. Similarly, qmail may have made deliveries successfully according to most of the .qmail file and then fail permanently. The SENDER is informed that the delivery failed, but not about at which point.
ezmlm takes advantage of these basic mechanisms to build a fast, efficient, and very configurable mailing list manager from a set of small independent programs.