`MessageSendingResponse` has incorrect `MessageID` type

[coreyward]

When the API returns a 406 error code it doesn't seem to include a MessageID, yet MessageSendingResponse has it typed to always be a string. This lack of alignment between the types and the runtime behavior caused a bug in production—our code had been expecting the MessageID to be a UUID in all cases.

It seems like this library is largely abandoned, and Postmark itself hasn't seen much in the way of attention in the last several years. Is there some other recommendation for transactional email from ActiveCampaign, or should I be looking at the competition?