It appears that kmail requires akonadi to provide a few extra Agent Types.

as of kmail 23.04.3 the following are listed as dependencies

  * akonadi
  * akonadi-contacts
  * akonadi-mime
  * akonadi-search

installing the following packages enables kmail to start up. The import 
& account wizards are still unavailable so you may need to add any 
account settings manually.

  * akonadi
  * akonadi-calendar
  * akonadi-contacts
  * akonadi-mime
  * akonadi-notes
  * akonadi-search