It seems you have a syntax issue in your JSON request.Look closely after content you have an extra closing } which means you are not posting 'toRecipients' in the message.

{"message":{"subject":"hellowolrd","body":{"contentType":"text","content":"wow"}},"toRecipients":{"emailAddress":{"address":"example@yahoo.com"}}}

Instead try below JSON to make it work - { "message": { "subject": "hellowolrd", "body": { "contentType": "text", "content": "wow" }, "toRecipients": [ { "emailAddress": { "address": "example@yahoo.com" } } ] } }

Thanks!

The invalid recipient was caused by a different json structure. I had to exactly match the following structure that was provided in one of their example.

 $mailArgs ='
{
    "message": {
        "subject": "Meet for lunch?",
        "body": {
            "contentType": "Text",
            "content": "Lunch Invited"
        },
        "toRecipients": [
            {
                "emailAddress": {
                    "address": "example@example.com"
                }
            }
        ]
    }
    }';

So I created a JSON array to match exactly the same structure and it worked!