I believe there is a bug with the SendMMS functionality when the "From" parameter is provided.
When sending a message crafted from the example here: https://dev.telstra.com/content/messaging-api#operation/sendMms a response is returned stating that the message delivery status is "MessageWaiting', however it never arrives.
If the "from" parameter is removed, the message is received within seconds.
I've tested via Postman and via the .NET SDK with the same results.
The value I'm providing for the parameter meets the requirements of not being greater than 11 characters and is purely alphabetical. This value works without issue when sending a regular SMS.
Thanks very much.
Example of a failing request:
Example of a successful request: