If SENDER is not defined ezmlm-issubn exits with an error.
If SENDER is on [any of] the mailing list[s], ezmlm-issubn exits with a zero exit code.
If box@domain is not on the mailing list, ezmlm-issubn exits 99. This exit code is non-success from a shell point of view, but to qmail it means "success and skip remaining lines in the .qmail file). Thus, a simple way to execute a delivery if the criteria are met is to place the ezmlm-issubn line first, followed by the action line. If SENDER is a subscriber, the action line is executed, if not, the line is ignored without the generation of an error condition. To generate a fatal error, just:
|/path/ezmlm-issubn dir ... || (echo "err msg"; exit 100)
If dir1 (or more) are present on the command line and are relative paths, ezmlm-issubn only examines the named subdirectories within the preceding absolute path dir.
ezmlm-issubn exits 100 on permanent and 111 on temporary errors.