KMail2 doesn't store information in databases

Same as in KDE3. Actually, probably not. In KDE3 a KMail crash could corrupt your database. In KDE4, well, I think it's harder to corrupt MySQL than the home-brewn database from KMail1 so I'd say it is more resilient to crashes and such.

And I'm assuming here that the *backend* of KMail2 crashes. KMail1 had everything (UI, database, plugins for mail retrieval etc) lumped together in one process, KMail2 has the backend abstracted away in a proper, C++ only, clean system with plugins for retrieval, caching and storage of data.