I agree, but you could try slicing it thinner (as thin as you can) and that should help.
I soaked the corned beef in water for a few hours prior to smoking. After applying the pepper/coriander seed rub, I smoked it @ 250F until it was 165F, internal temp. Foiled it until it reached 180F, internal temp. I let it set on the counter until it cooled, and then put it into the fridge overnight. I've been eating it reheated in a toasted cheese sandwich on rye. It is tasty, but on the tough side. Assuming it wasn't just a tough piece of meat to begin with, what could I have done differently to make it more tender?
Sounds like it wasn't cooked long enough. How long was it in the smoker?
Do what the best traditional delis like Carnegie, Langers, Katz's, and others of that dying breed do: they steam the meat for 2-3 hours
or more (that's AFTER it's been smoker cooked) . The smoking and heating to 165° may technically cook the meat, but the melt-in-your-mouth succulence of properly prepared pastrami just won't happen without that long steambath.
The great thing is that once the meat has been long-steamed this way it can be refrigerated afterwards and sliced as needed...and at that point, whether it's served cold or reheated, it will be as tender as can be.
As far as I'm concerned, it's the ONLY right way to do pastrami. Even the marginal supermarket deli brisket pastramis can be made at least palatable and tender with this method.
But if you're curing and smoking your own pastrami from scratch, just don't make the mistake of going for a lean first-cut brisket...use the fattier 'second cut' one or better still, use the traditional cut of meat for pastrami...the beef 'navel'.
Bottom line: Lean meat just doesn't cut it for real pastrami.
Even if you start with a packaged corned beef to which you intend to add a coriander/pepper rub prior to smoking, don't go for the thin cut brisket corned beef, go for the thick or 'point' cut.
Your arteries may not thank you, but I promise your taste buds will.