{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "$id": "https://smtp.standard/schemas/message",
  "title": "SMTP Email Message",
  "description": "Schema representing an email message as defined by RFC 5321 (SMTP) and RFC 5322 (Internet Message Format)",
  "type": "object",
  "required": ["from", "to", "subject"],
  "properties": {
    "from": {
      "type": "string",
      "format": "email",
      "description": "The sender's email address (RFC 5322 From header / SMTP MAIL FROM)"
    },
    "to": {
      "oneOf": [
        {"type": "string", "format": "email"},
        {
          "type": "array",
          "items": {"type": "string", "format": "email"},
          "minItems": 1
        }
      ],
      "description": "Recipient address(es) (RFC 5322 To header / SMTP RCPT TO)"
    },
    "cc": {
      "oneOf": [
        {"type": "string", "format": "email"},
        {
          "type": "array",
          "items": {"type": "string", "format": "email"}
        }
      ],
      "description": "Carbon copy recipient(s)"
    },
    "bcc": {
      "oneOf": [
        {"type": "string", "format": "email"},
        {
          "type": "array",
          "items": {"type": "string", "format": "email"}
        }
      ],
      "description": "Blind carbon copy recipient(s) - not included in message headers"
    },
    "replyTo": {
      "type": "string",
      "format": "email",
      "description": "Reply-To address if different from From"
    },
    "subject": {
      "type": "string",
      "description": "Email subject line (RFC 5322 Subject header)"
    },
    "date": {
      "type": "string",
      "format": "date-time",
      "description": "Message creation date and time (RFC 5322 Date header)"
    },
    "messageId": {
      "type": "string",
      "description": "Unique message identifier in format <local@domain> (RFC 5322 Message-ID)"
    },
    "body": {
      "$ref": "#/definitions/MessageBody"
    },
    "headers": {
      "type": "object",
      "description": "Additional RFC 5322 headers as key-value pairs",
      "additionalProperties": {"type": "string"}
    },
    "attachments": {
      "type": "array",
      "description": "MIME attachments",
      "items": {
        "$ref": "#/definitions/Attachment"
      }
    }
  },
  "definitions": {
    "MessageBody": {
      "type": "object",
      "description": "The message body content",
      "properties": {
        "text": {
          "type": "string",
          "description": "Plain text body (text/plain MIME part)"
        },
        "html": {
          "type": "string",
          "description": "HTML body (text/html MIME part)"
        },
        "mimeType": {
          "type": "string",
          "description": "MIME content type for multipart messages",
          "examples": ["multipart/mixed", "multipart/alternative", "text/plain", "text/html"]
        }
      }
    },
    "Attachment": {
      "type": "object",
      "required": ["filename", "contentType"],
      "properties": {
        "filename": {
          "type": "string",
          "description": "Attachment filename"
        },
        "contentType": {
          "type": "string",
          "description": "MIME content type (e.g., application/pdf, image/png)"
        },
        "contentId": {
          "type": "string",
          "description": "Content-ID for inline attachments"
        },
        "disposition": {
          "type": "string",
          "enum": ["attachment", "inline"],
          "description": "Content disposition type"
        },
        "size": {
          "type": "integer",
          "description": "File size in bytes"
        }
      }
    }
  }
}