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Ok, it seems that I needed to add a new trait to my user schema to hold these verifiable addresses. This worked:

{
  ...
  "properties": {
    "traits": {
      "type": "object",
      "properties": {
        "username": {
          "type": "string",
          "<http://ory.sh/kratos|ory.sh/kratos>": {
            "credentials": {
              "password": {
                "identifier": true
              }
            }
          }
        },
        "email": {
          "type": "string",
          "format": "email",
          "<http://ory.sh/kratos|ory.sh/kratos>": {
            "credentials": {
              "password": {
                "identifier": true
              }
            },
            "verification": {
              "via": "email"
            },
            "recovery": {
              "via": "email"
            }
          }
        },
        "verification_emails": {
          "type": "array",
          "items": {
            "type": "string",
            "format": "email",
            "<http://ory.sh/kratos|ory.sh/kratos>": {
              "verification": {
                "via": "email"
              }
            }
          }
        }
      }
    }
  },
  "additionalProperties": false
}

This is explained here for recovery addresses, which work the same way.