If you are looking to obtain a domain name that matches your trademark, you have to ask yourself several questions. The first is whether or not the person who has the domain name is intentionally violating your trademark. If the person owned the domain name before you established trademark rights, or achieved the level of fame where they might know you exist, then you are going to have to purchase the domain name from them. If the person registered the domain name in order to divert traffic because of your trademark, then they may be a cybersquatter.
For domain names that are registered in violation of either the Uniform Domain Name Resolution Policy (UDRP) or the Anti-Cybersquatting Consumer Protection Act (ACPA), you have a couple different options. You can file a UDRP arbitration or to seek a transfer of the domain name to your control. Under the ACPA, you could potentially file a lawsuit or send a threat letter arguing the person violated the statute and should transfer the domain name to you. Under the ACPA, you could potentially obtain attorney's fees and damages for trademark infringement. Under the UDRP you are limited to a transfer of the domain name to your control.
You should not be sending out any threat letters alledging trademark violations unless you have thouroughly analyzed the history of the domain name using the WhoIs Database. Even then, it requires a level of sophistication to determine if the person has held the domain name for a long time and thus predates your trademark rights. You should contact a cybersquatting or domain name lawyer in order to better understand your options and rights. You don't want to send a threat letter alledging trademark violation if you are ultimately going to have to purchase the domain at fair market value. Sometimes you have to decided whether or not you are going to lead with cookies and ice cream rather than blunt force trauma.