Usually an RDBMS like PostgreSQL is used in an environment that has a different usage pattern than search. SOLR can take advantage of specializing for that type of usage. However, Russia's largest search engine, Yandex, seems to like PG http://momjian.us/main/blogs/pgblog/2016.html#September_28_2...
I doubt they use it for actual search, though. Given that core company product for end-users is Search Engine, Yandex probably has some in-house system, that Mail team can leverage for their searching needs.