| Name | Type | Description | Notes |
|---|---|---|---|
| attachment_id | str | When present, contains the ID of an external attachment that can be retrieved in a separate messages.attachments.get request. When not present, the entire content of the message part body is contained in the data field. | [optional] |
| data | str | The body data of a MIME message part as a base64url encoded string. May be empty for MIME container types that have no message body or when the body data is sent as a separate attachment. An attachment ID is present if the body data is contained in a separate attachment. | [optional] |
| size | int | Number of bytes for the message part data (encoding notwithstanding). | [optional] |