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4.37 How bounces are handled

Ezmlm messages are sent with an envelope sender (‘Return-Path’) that directs bounces to DIR/bouncer and also via “VERP” contain information about the intended recipient. Thus, programs run from DIR/bouncer know the subscriber for whom the message bounced. ezmlm-weed(1) is used to weed out delivery delay notification and other junk. For others ezmlm-return(1) decides if the address is a subscriber. If so, it saves the first bounce message and a list of bounced-message numbers. ezmlm-warn(1) executed from e.g. DIR/editor goes through these bounce files. If it finds any that are older than 1,000,000 seconds (about 11.6 days) it sends a warning message to the subscriber. If this warning message bounces, ezmlm-return(1) sets up a “warning flag” for the subscriber. If ezmlm-warn(1) finds a warning flag older than 11.6 days, it sends a "probe" to the subscriber. If ezmlm-return(1) receives a bounced probe, the subscriber is automatically unsubscribed.

The ezmlm-warn(1) -t switch can be used to change the time-out (in days). The ezmlm-warn(1) -d switch causes processing of ‘list-digest’ bounces rather than ‘list’ bounces. ezmlm-weed(1) and ezmlm-return(1) can handle bounces for either list.

ezmlm-warn(1) also removes any files in the bounce directory that are older than 3 times the bounce time-out.

ezmlm-warn(1) is normally run from DIR/editor. This can take quite a lot of resources, if there are a large number of bouncing addresses (>>1000) on a busy list, since by default all bounces are stored in a single directory and ezmlm-warn(1) examines all of them with each invocation. ezmlm-idx->=0.32 changes bounce handling to improve performance for large lists. Bounces are stored in subdirectories of DIR/bounce/d/, one per 10,000 seconds. The corresponding address hashes are stored in 16 subdirectories of DIR/bounce/h/. Instead of looking at all bounces, ezmlm-warn(1) processes only the bounces in DIR/bounce/d/ subdirectories that are “due”. In addition, ezmlm-warn(1) uses DIR/bounce/lastd as a simple lockout, to assure that it will do work only at most once every 5.5 hours. (Times are scaled to the ezmlm-warn(1) -t argument if used.) Together, these changes assure that bounce handling will scale well in the default configuration, even for very large lists.