We found a way to resolve this by making the uddeim inbox menu link Public (instead of Registered). It is an invisible menu anyway, so I don't think user security is compromised (your opinion?). Now the uddeim link stays at the uddeim url and the user is able to log in and go directly to their message.
If the user is not logged in, they see "You are not logged in." on the page, if they log in as the wrong user, they see: "This message cannot be displayed.
Possible reasons:
You have no rights to read this message.
The message has been deleted."
If they log in as the right user, they immediately see their message.
Seems like the best result.