ezmlm uses the concept of sublists. Sublists are regular ezmlm lists, except that they only accept messages from their parent list, which is placed in the file DIR/sublist.
sublists are used to split the load of a large mailing list among several hosts. All you need to do to set up a local sublist of e.g. the ‘email@example.com’ list is to create a ezmlm list, and put ‘firstname.lastname@example.org’ into DIR/sublist of you list, and subscribe the sublist to the main qmail list. Now anyone can subscribe to your local list which handles its own bounces, subscribe requests, etc. The load on the main list is only the single message to your local list.
Sublists will not add their own mailing list header and they will not add a subject prefix. Normally, sublists will use their own message number, rather than that used by the main list. With ezmlm-idx>=0.23, sublists that are not archived and not indexed, will instead use the main list message number. This way, bounce messages from the sublist can refer the subscriber to the main list archive. This is not done for indexed/archived sublists for security reasons (an attacker could overwrite messages in the sublist archive).
With ezmlm-idx>=0.31, there is support for using ezmlm as a sublist of a mailing list run by another mailing list manager. To set this up, set up a normal ezmlm sublist, then edit DIR/editor so that the ezmlm-send line contains the command line option -h X-Listprocessor-Version: (before DIR). As the header text, you need to use a header that the main list manager adds to messages. Now your sublist will accept only messages from the main list requiring that they come from that list and contain the header specified.
ezmlm-idx>=0.313 also has added protection against the malicious subscription of the ezmlm list to mailing lists run by other list managers. If the ezmlm-reject(1) line in DIR/editor has -h and ‘DIR’ on it, ezmlm-reject(1) will read DIR/headerreject and reject messages that have any header specified in that file. See the ezmlm-reject(1) man page for suitable headers.