'Tis the season to be suing.

A day after it formed a licensing-and-investment pact with mobile email patent holder NTP, closely held mobile email shop Visto says it has filed a patent infringement complaint against Microsoft ( MSFT).

The Redwood Shores, Calif.-based company says Microsoft's Windows Mobile 5.0 system uses multiple patents related to wireless email and data access that it developed. Visto says it has filed a lawsuit in U.S. District Court for the Eastern District of Texas seeking an injunction to stop distribution of Microsoft's smartphone operating system, along with cash for damages, according to a Visto press release.

Microsoft says it hasn't reviewed the complaint, but adds that the company "stands behind its products and respects the intellectual property rights of others."

The Visto challenge is only the latest patent dispute in a rapidly growing and highly contentious wireless email industry.

Visto, an upstart in the email-to-cell phone sector, bolstered its intellectual property strength this week by teaming with NTP, the disputed holder of wireless email patents central to Research in Motion's ( RIMM) BlackBerry service. On Wednesday, NTP agreed to license patents to Visto, and as part of the deal, NTP would take an ownership stake in the company.

In addition to Visto, NTP now has licenses with mobile email rivals Good Technology and Intellisync ( SYNC), which agreed earlier this year to be acquired by Nokia ( NOK).

Meanwhile, investor confidence in mobile email giant RIM has been slipping as a four-year legal battle with the Virginia patent shop intensifies. RIM shares have dropped 25% this year as the momentum builds behind NTP's claims.

Last month, the Waterloo, Ontario, BlackBerry maker was dealt another setback when a judge tossed out RIM's attempt to hold NTP to a $450 million settlement that fell through in June. Now, some analysts say RIM will be fortunate if it can reach a settlement that costs less than $1 billion.

Observers expect RIM to pursue a settlement, but the company says it's got other options. RIM is seeking another appeal hearing with the Supreme Court and is awaiting a ruling from the U.S. Patent and Trademark Office. The company has also been working on a so-called software workaround that would allow the service to operate without using the contested patents.

In the past week there has been speculation that RIM is preparing a couple approaches to a workaround, should it be forced to use it.

Among the solutions is an email alert that sends a wireless message to a device with the basic information about the email like sender, subject, time and date. The current BlackBerry service sends most or all of the email. NTP has argued that it has a patent on the "push" technology that sends email to your office inbox and your wireless device simultaneously.

This particular workaround, which presumably involves only a software change at RIM's network center, would send users just the header or identifying information about the email and let users retrieve the email if they wanted it.

A RIM representative declined to comment on any specifics of the workaround, and questioned the validity of recent speculation.

"We have not yet disclosed workaround details due to confidentiality and legal strategy considerations, but we are actively considering a communications plan and timing," the rep said in an email.

But skeptics remain unconvinced.

"What I don't understand is, if they have a workaround solution ready, why don't they use it?" says Ovum analyst Roger Entner.

Entner has not seen any of the proposed workaround solutions, but says based on the description of the "push" header and pull email, the system could possibly sidestep the patent problem.

"It's not elegant, says Entner, "but it is certainly cheaper than a big settlement."