From Mousesavers...
"If you are a repeat resort guest and have already subscribed to a lot of Disney lists, it's possible you have received a discount e-mail but it went into your "spam" folder or was filtered out as junk mail by your e-mail provider. However, if you are in the Disney reservations system, the discount information should be associated with your name. For that reason, when you call the Disney Reservations Center to make room reservations, it's worthwhile to make a polite inquiry. You might say something like "Would you mind checking to see if I am on the list for any room discounts? I'm afraid I may have missed a promotional e-mail." Don't bother calling about this if you have never stayed on Disney property before -- you almost certainly won't be in the system."
I would think that this would apply to a code that had been snail-mailed, as well, since anything can happen to a little post card in transit!
Hope this helps!