Associate Email API Internal Error

I am trying to associate an already existing email within a function using the Related Emails API.

To provide more context, I also have admin permissions and have ensured that the fields are correct and that I have admin permissions when associating the email.

However, I am getting the following response:

"RELATION RESPONSE {"code":"INTERNAL_ERROR","details":{},"message":"Sorry, we are unable to process your request. Please try again later.","status":"error"}"

Here's a preview of the Json (with some placeholders to avoid private info) that I'm sending to the API.

{"Emails":[{"from":{"user_name":null,"email":"email.com"},"to":[{"user_name":"20GS008737","email":"email.com"}],"subject":"EMAIL SUBJECT","content":"EMAIL CONTENT","rich_text":true,"mail_format":"html","org_email":true,"date_time":"2024-12-13T09:21:36-05:00","sent":true,"original_message_id":"MESSAGE_ID","linked_record":{"id":"RECORD_ID","name":"Owner ID","module":{"api_name":"Case_Processing","id":CASE_ID}},"owner":{"id":"OWNER_ID"}}]}

Very grateful for any help you can give me.