Xapian
Uses the popular open-source Xapian library to index
FTS Flatcurve
Dovecot (2.3.x) Full Text Search (FTS) Plugin
An easy-to-setup Dovecot plugin that uses Xapian to locally index messages
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Local Search
Indexing is done on the Dovecot backend - no external service needed
RFC Compliant
Supports match scoring and substring matches, making it RFC 3501 (IMAP4rev1) compliant (although substring searches are off by default)