> The other obvious option is to shard the mail folders to ensure there's no mor...

> The other obvious option is to shard the mail folders to ensure there's no more than X files in each folders, but that becomes pretty complex IMHO.

I mean it depends on the volume of e-mail you get, but:

  INBOX
  Archive/2004
  Archive/2005
  ...
  Archive/2023

Isn't something I would call complex, and works out to about 10k files per directory for me.