Notmuch is an answer to Sup. Sup is a very good email program written by
William Morgan (and others) and is the direct inspiration for Notmuch.
Notmuch began as an effort to rewrite performance-critical pieces of Sup in
C rather than ruby. From there, it grew into a separate project. One
significant contribution Notmuch makes compared to Sup is  the separation
of the indexer/searcher from the user interface. (Notmuch  provides a
library interface so that its indexing/searching/tagging features can be
integrated into any email program.)

This requires xapian-core.