In order to obtain the TAU, you had to get 100 Nanotechs from the Wheel of Fate (Roulette) that was up dated for this. You will not have one in your inbox, you had to obtain the gun. As for when the gun expires in your inbox (if you had a relative or something obtain it) would be January 31st.
I have a spare 30 in my inbox after I made them all (sb, Combat, CQB and tau)
At this point I'd say it's impossible to even obtain the TAU if dont have a shit load of silver coins. I'm sure you could give someone on MPGH or you know your account information if you're absolutely desperate.