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.