espocrm-base/application/Espo/Modules/Crm/Resources/metadata/entityDefs/Email.json

{
    "fields": {
        "account": {
            "type": "link",
            "readOnly": true
        },
        "parent": {
            "entityList": ["Account", "Lead", "Contact", "Opportunity", "Case"]
        }
    },
    "links": {
        "account": {
            "type": "belongsTo",
            "entity": "Account"
        }
    }
}