> Search is a very different story, you wouldn't want to have to do a full directory scan for text based search. So some level of indexing would be useful for a client mail service.

I don't know; my ~/code directory has tons of stuff and searching with ripgrep doesn't seem too slow:

  % time rg HelloWorld | wc -l
  4
  rg HelloWorld > /dev/null  0.13s user 0.12s system 99% cpu 0.251 total

  % time rg string | wc -l
  57813
  rg string > /dev/null  0.20s user 0.14s system 99% cpu 0.339 total

Rough estimate of files that rg will search:

  % scc
  ───────────────────────────────────────────────────────────────────────────────
  Language                     Files       Lines     Blanks    Comments      Code
  …
  Total                        11024     1864982     175565      208777   1480640
  ───────────────────────────────────────────────────────────────────────────────

Finding close to 60k matches in 11k files/1.7M lines in about 0.3 seconds isn't too bad.

It should be said I ran a few commands on that directory before the above results, so there's probably some filesystem caching going on, but I can't be bothered to reboot.

For many (not all, obviously) cases I think you may be able to get away without a index. Most people aren't subscribed to tons of email lists and get maybe a few emails a day at the most.

I'd consider anything below ~3 seconds to be fine for search, so this scales to about 100k files/emails. At 10 emails/day on average that's about a decade. Most people do not get 10 emails/day on average.

And you can even do some "poor man indexing" by just making a new directory every five or ten years. Most of the time you want just emails from the last year or so.