"According to this Microsoft article the root cause of the problem is a change made by Kaspersky to the format of their signature downloads. The Kaspersky engine is one of the engines included with Forefront, and the signatures are downloaded from Microsoft.com. The change has caused a compatibility problem with Forefront due to the way in which Forefront interprets file names that start with a period character."
http://exchangeserverpro.com/kapersky-scan-engine-updates-for-forefront-causing-high-volume-of-downloads
"According to this Microsoft article the root cause of the problem is a change made by Kaspersky to the format of their signature downloads. The Kaspersky engine is one of the engines included with Forefront, and the signatures are downloaded from Microsoft.com. The change has caused a compatibility problem with Forefront due to the way in which Forefront interprets file names that start with a period character."