DEFAULT_PROBE

A feature called passive probing is enabled by default for lists under a certain size. This variable controls two aspects of passive probing. The first parameter to this variable is the length of the default passive probing cycle in days. Because passive probing is very resource-intensive, it is disabled by default above a certain list size. The second (and optional) parameter to this variable is the list size beyond which passive probing is disabled unless you explicitly enable it in the Auto-Delete list header keyword.

Since probes do not work with Unix-based mail programs without reconfiguration (or, specifically for sendmail, recompilation) to accept the LISTSERV address probe format, the default under Unix is to disable passive probing altogether.