[jfmc]

Xapian is used in https://www.djcbsoftware.nl/code/mu/ for indexing emails.

[calvinmorrison]

And by Fastmail

[nathell]

And by Notmuch https://notmuchmail.org/

[Bluestein]

In itsel, Notmuch, a very interesting tool.-

[Bluestein]

s/itsel/itself/