I would make sure you filed a police report on it and say the managet threated you also, then go in there and get a sandwich and poison it. When they find you dead, the police should put 2 and 2 together and those bastards will rot in jail!
Ok well maybe not.....
On a serious note subways are franchises so most of them are individually owned. I don't now what effect this would have on going higher up in the corporate ladder. I also believe that the e-mail could of been intercepted by a shift manager or another person. Guess you just need to decide how much closure you want on this matter as you should of already decided to never go back to that location.