received a letter about trademark infringement, what should we do next?

The short answer is "yes, you can be sued for trademark in infringement." Whether the other party has any chance of succeeding is another question entirely. People don't need to have a chance to win to sue you. Sometimes they may think they do, so for the sake of argument lets give them the benefit of the doubt in terms of integrity. They claim to have a trademark that is similar to your domain name. Even without federal registration of their company's name they can still have a common law trademark, which gives them the right to protect their mark. You are right to pick up on the fact that trademarks protect against use of a confusingly similar mark in related areas of commerce. You may be 100% that your businesses are in entirely unrelated fields. If we once again give them the benefit of the doubt, they may simply not know that. Perhaps, from their point of view, they see a website with a name that is pretty similar to their company's name. They may not have looked any further than that.

They may be attempting to protect their company's trade name from infringement. You may be absolutely sure that you are not infringing. Even so, you should probably take it seriously enough to talk to an attorney, give them the details, and sort this out quickly and cheaply. Otherwise it may simply go away, or it may not. If you're right, put the issue to rest so you don't have to worry about it and move on.