The anonymous letter contained a handwritten note saying the authors knew where speaker Laura Boldrini lived and threatening to throw acid in her face, and was signed with a five-point star, according to Italian media reports.
Police said the handwritten letter, complete with spelling mistakes, was intercepted by postal workers in north Milan, but the author was unknown.
Investigators said they ruled out the hand of Italy's Red Brigades, a notorious extremist group formed in the 1970 which used a five-point star logo.
News of the letter sparked reactions of censure from across the political sphere and support for Boldrini, who was the target of a flurry of verbal attacks this week from members of the anti-establishment Five Star movement.
She accused the movement on Monday of instigating violence and slammed bloggers on the party website as "potential rapists", after a satirical post by Five Star leader Beppe Grillo sparked a flood of abuse against Boldrini.
The post came in the wake of a fall-out between the Five Stars and Boldrini over a controversial measure aimed at shoring up Italy's public accounts.
Last week Boldrini used her so-called "guillotine" powers to cut short a filibuster by M5S deputies attempting to block the adoption of the decree, which passes part of the cost of scrapping a housing tax onto Italy's banks.
After being elected last year, she also called for a new law to defend Italian women against violent acts.