Ok
Using the PHP interface I can sent a message with a usercontext field and when I redirect the reply to an email address I see this value on the subject line - great.
However that is all well in good but the user context is NOT shown on the delivery status email for the same message and so if I sent two message with two different topics to a number and 1 was not delivered I would not be able to tell which of the two messages was the undelivered one.
So how do I get the usercontext field to be shown on the delivery status emails?