I'd initially take a look at the following:
It could be something simple like no user account exists for the mail address being used to send an email to LDSD
I agree with Dave. All emails processed by Inbound Mail need to match to an email address used in one of the user profiles in Service Desk. If no match is found the Incident will not be logged; that said the system will usually send that user back an email reply via Outbound Mail saying something like "You are not a valid user of the system!"