ezmlm subscriber email addresses are stored within DIR/subscribers/ as a hashed set of 53 files. The hash calculated from the address determines which of the 53 files and address is stored in. Thus, to find out if an address is a subscriber, ezmlm has to read at most about 2% of the email addresses. The hash function insures that email addresses are reasonably evenly distributed among the 53 files.
Addresses in the files in DIR/subscribers/ are stored as strings starting with ‘T’, followed by the address, followed by a zero byte. This is the same format as qmail-queue(8) uses for its list of recipient addresses. Thus, subscriber lists can be directly copied to qmail without any further processing.
With ezmlm-idx version 0.32 and later, you can use an SQL server for the subscriber databases. Please see the SQL section (ezmlm support for SQL databases).