post https://sandbox.datamotion.com/SMCApi/v3/content/messages
HTTP Status Code 400 Error Codes (error property):
InvalidRequest - The request is invalid (the provided message provides further detail).
RecipientNotSpecified - At least one value, in the To field, was not specified (required).
SubjectExceedsMaximumLength - The subject is too long. The maximum length is 255.
InvalidAttachmentContentId - All contentId field values, of inline attachments, must contain an @ symbol.
CrmCustomIdExceedsMaximumLength - The attributes->crmCustomId value is too long. The maximum length is 255.
HTTP Status Code 401 Error Codes (error property):
Unauthorized - An Authorization token was not provided and is required.
AccessTokenInvalid - The access token is expired or invalid.