First get a lawyer. If and only if your lawyer tells you to send the other party a letter; send them a letter. You will most likely need to apply for the Tradem

First get a lawyer. If and only if your lawyer tells you to send the other party a letter; send them a letter. You will most likely need to apply for the Trademark protection you are wanting to enforce before you do so. This is one of the areas where winging it is not an option. If you communicate with them in any way you run the risk of creating evidence that your claim to the trademark is not as strong as it should be and you could unknowingly give the other party what they would need to force YOU to change your company's name.