Software Copyright Infringement

We are a software company which is registered in India. However, we have used U.S. Mailing Address as our contact info on our site, as most of our clients are from US. One of our competitor software company which is registered in US have send us attorney's letter stating that we have violated US copyright law 17 USC § 101 and misappropriation of intellectual property is done and if company proves the infringement of copyright they will be entitled to get the damages suffered and demanding us to cease and desist operation of our company. We have similar kind of software but technology and code are totally different, but some portions of user interface look similar. I have few questions:

1. We have already applied for copyright in India, but we are not sure if the other company is having copyright for their software. How can we check copyright status of their software? and what if they don't have copyright registered for their software?

2. Can US company send such letter to an Indian company? Or is it because contact info on our site is US address they are able to send us such notice? It is just a mailing address and all our operations are done from India and providing online software service in US. What if we change our contact info to an Indian address on our site?

3. What should be our response to their letter considering above 2 points.

1) Copyright is automatic, although they likely did register their copyright even if not they still have copyright, just less options for enforcement until they do register.

2) US courts will have jurisdiction over you, you do business in the US.

3a) First do a quick investigation to ensure none of your employees did in fact copy their code.

3b) as long as you did not copy their code or resource files you can ignore the letter or have your lawyer draft a short letter denying infringement.

Source code is completely different and even software is developed in different technology. They are claiming that user interface is similar. Some screens of our software may be having similar look in terms of field names, but we have used alternate field names, e.g. if they have 2 fields for first name and last name, we have used single field Full Name. Can someone please elaborate on User Interface copyrights, we have also used different design, layout, colors and alternate field names are used.