The expire plug-in is typically used to optimize expunging of old messages from users’ mailboxes. When enabled, it reduces disk I/O. The plug-in should be enabled globally - i.e., for all mail plug-ins rather than only specific protocols.
Configuration for the expire plug-in’s MySQL back end is handled in dovecot.conf, with a setting such as the following in the plugin block:
expire_dict = proxy::expire
A setting of yes (the default) enables caching of the dict value in the dovecot.index file. This setting for the expire plug-in significantly reduces the number of dict look-ups.