A company has the domain name www.domain.com which redirects to a totally different domain?

a company has the domain name www.domain.com which redirects to a totally different domain www.differentsite.com and they do not have www.domain.com registered as a trademark. How legal is it for me to create a domain www.xdomain.com and register xdomain.com as a trademark given that it is quite similar to thier domain name?