FineE said:
It may well be worth this fight. It all depends. One thing I would not do is contact the company and give them the heads up without getting legal advice.
One thing to keep in mind here is that this "big company" with "millions of dollars" to pay lawyers did not manage to oppose the trademark application. Yikes! Did you file your TM in the same jurisdiction where this company is in business? For related goods or services? Or are we talking a TM in an obscure jurisdiction for unrelated goods and services?
To say this is clear as mud is an understatement. I will certainly suggest professional legal advice.
Simple facts are that if the company was using the name before, then they have whats called common law trade mark. And if they can prove that they used the TM prior to our friend here then really it is an open and shut case.
Common law TradeMark protects you as far as someone would have to take you to court to prove infringement having the trademark registered automatically does give you the right to sue for trademark infringement and also allows you to sue for damages, of course would have to have sufficient grounds to claim infringement, however the fact that they had common law trademark prior to your application for registered trademark they would have the law on there side.
I would put this down to experience, and move on
