Terms of the deal were not disclosed.
Microsoft said the acquisition will allow it to offer a service for enforcing compliance through archiving, reducing spam and viruses, and ensuring email availability in the event of a disaster.
Dave Thompson, corporate vice president of the Exchange Server Group at Microsoft, said FrontBridge's services are "a perfect complement to Exchange."
In a statement released Wednesday, email security suppplier Postini said Microsoft's acquisition validates its managed services strategy.
"The acquisition fits with Microsoft's typical approach to acquiring a secondary player to accelerate time to market," the company said.
The FrontBridge deal is the latest security purchase for Microsoft. Last month, it finalized its purchase of antivirus supplier Sybari Software.