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