Recipients of email (To, Cc, Bcc) should be emailable.

This mean do not offer non-emailable entities, like Knowledge articles, Cost estimations, but also Contacts or Accounts, if there is no email available, in To, Cc, Bcc fields.

There is no setting for showing or hiding entities in Email form, Recipient control.

Especially the New Enhanced Recipient control, when displaying Recent records might be confusing, displaying inappropriate results.

With old Recipient control, the knowledge article records are searchable too, but hidden behind a number of steps.

Full video for behaviour was uploaded to ticket ID#2507021420000870.