I should first recommend you call the DMV at (800) 777-0133 and ask them why
you need to smog check your Ford Mustang again. It may be that someone noted
your Mustang's license plate and reported it to the DMV as a smog check
problem. This may or may not be true, however the DMV will require you get
your Ford smog inspected again. I should point out however that typically in
a complaint type of situation it's the BAR (Bureau of Automotive Repairs)
who contacts the vehicle's owner, requesting a smog check, and not the DMV.
It's a bit confusing in this case. Your best bet is to call the DMV to get
to the bottom of it. Hopefully they've made a simple mistake and will
retract their smog inspection requirement... Specially since your Ford is
due for a renewal until August.
I should note this as well (and this is what I think is happening)... If
your Ford was smog checked in January because you purchased it then, or
there was a title transfer involved, that smog certificate will not have
anything to due with the smog check certificate due when the vehicle's
registration renewal period arrives. Remember registration renewals are sent
out 3 months in advance. This is what may have happened in your case.