POST /messaging/v1/email/messages/{messageKey}

Sends an email to a single recipient using a send definition.

Name	Type	Description
`messageKey`	string	Required. A unique identifier that you can use to track the status of the message. The key can contain up to 100 characters. There are no character restrictions. The message key that you provide must be unique among all of the keys used in your business unit over the last 2 days.

Name	Type	Description
`definitionKey`	string	Required. The ID of the send definition.
`recipient`	object	Required. An object that contains parameters and metadata for the recipient, such as send tracking and personalization attributes. Use this parameter to send to one recipient. If this object is present in the request, the `recipients` array, which is used to send messages to multiple recipients, can’t be included in the request.
`recipient.contactKey`	string	Required. A unique identifier for the subscriber. You can create a contact key at send time if the contact isn’t already in Marketing Cloud Engagement.
`recipient.to`	string	Required. The recipient’s email address.
`recipient.attributes`	object	Personalization information for the message, provided as key-value pairs. The attributes must match existing profile attributes, content attributes, or triggered send data extension attributes.

The response includes a request ID. The responses array contains a messageKey value for the message. If a message was sent successfully, the object doesn’t contain any additional information. If a message failed to send, the response includes an errorcode that describes the reasons that the message failed.