I believe you can integrate mairix and notmuch with Dovecot and have it searched server-side. Client support lacking

Dovecot's server-side search is lucene based.

Unless you use the notmuch plugin, which is notmuch based.

Coo, I can't find that on the dovecot plugin wiki page, I imagine it's third party then :-)

notmuch can speak dsync and there's a third-party one yeah