Postini said it blocked more than 26 million DHAs in March, an average of 843,157 per day. That is a drop of 8 percent from the February average of 919,963 DHAs per day.
In a DHA, spammers connect to a business email server and guess email addresses in an attempt to find valid ones for spam campaigns.
The amount of spam and emails carrying viruses dropped slightly in March, according to Postini. Of the 14.6 billion messages the company processed this month, 87 percent were spam or malicious. In February, 88 percent of all email it processed was malicious.
"While the overall trend for the past five years has been a steady increase in email threats, we occasionally see small declines from month to month," Andrew Lochart, Postini's director of product marketing, said in a statement. "We foresee no long-term decrease in the amount of spam businesses can expect to receive."