It seems holding company NTP just loves to sue wireless companies. After winning $612.5 million from BlackBerry maker RIM with its controversial patents on wireless e-mail technology, NTP went after smartphone maker Palm at the end of last year using the same contested patents. Not finished, NTP is now suing U.S. wireless carriers that offer mobile e-mail. Will NTP ever stop?Here is a look at NTP's latest series of lawsuits:

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The lawsuits, against Verizon Wireless, Sprint Nextel Corp, T-Mobile USA and the mobile unit of AT&T were filed September 7 in the U.S. District Court for the Eastern District of Virginia, according to court documents.

NTP, which is asserting patents related to products, processes and services used for wireless e-mail systems, said it was seeking a jury trial, injunctive relief and monetary damages.

So far the carriers have yet to comment on the lawsuit. Apparently the lawsuit does not involve BlackBerry e-mail service offered over these carriers' networks, but centers on other mobile e-mail services.

NTP has emerged as a famous example of so-called patent trolls, holding companies that patent technology that they never actually use. These holding companies go after innovators, like RIM, who build successful businesses using technology that the holding companies claim as their own.

NTP's status as a patent troll is also an issue of debate. NTP was founded by the late Tom Campana, an entrepreneur who founded the holding company to protect his patents after his business failed.

Most of NTP's critics, however, claim that while Campana may have had interesting ideas, his patents actually bare only a slight resemblance to the wireless messaging systems that RIM, Palm, and carriers like Verizon Wireless, AT&T, and Sprint use to deliver mobile e-mail. This is why the U.S. Patent & Trademark Office rejected NTP's patent claims in 2005 during the middle of the NTP, RIM lawsuit. While NTP is still appealing the office's decision, many technologists claim that the office made the right call. Critics also point out that Campana ultimately failed and that the patent system needs to reward innovators like RIM that build successful businesses.

NTP's lawsuit against RIM helped make patent reform one of the hot button issues for the tech industry. If proposed reforms ever make it into law, companies like NTP would find it much more difficult to patent technologies they never built. It would also require greater specificity in patents, making it much harder for lawyers to patent broad concepts.

It also looks like NTP may not be as successful with Palm as it was with RIM. A judge in the NTP, Palm lawsuit granted a stay of the suit in March and struck down one of NTP's complaints against Palm.

What do you think? Will NTP's lawsuit against Verizon Wireless, AT&T, and Sprint fail? Or will NTP cash out once again?