As your documentation states, "In case of errors on one or several of the messages, the API will not stop the processing of other successful messages.". This is not reflected in the documentation, examples, response types or error types and can easily lead to wrong usage and sending mails multiple times.
Please define and document a type for the response of sending a mail. Currently, Promise<LibraryResponse> is returned which is completely useless.
For batch usage, it is mandatory to include a try / catch and evaluate the response of both successful API calls as well as errors. Nothing in the client documentation hints that even though an error is thrown, mails have been sent.
As your documentation states, "In case of errors on one or several of the messages, the API will not stop the processing of other successful messages.". This is not reflected in the documentation, examples, response types or error types and can easily lead to wrong usage and sending mails multiple times.