'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.

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