The volume of spam has dropped steadily since August, but the Christmas period saw a precipitous decline.
One security firm detected around 200 billion spam messages being sent each day in August, but just 50 billion in December.
Spam Levels Fall Suddenly
While the reasons for the decline are not fully understood, spam watchers warn the lull may not last.
Around the Christmas holidays, three of the largest spam producers curtailed their activity, Paul Wood, a senior analyst at Symantec Hosted Services told BBC News.
"But it's hard to say why," he added.
The vast majority of spam is sent by networks of infected computers known as botnets.
One of these botnets, known as Rustock, was at its peak responsible for between 47% to 48% of all spam sent globally, said Mr Wood.
Spam Levels Fall Suddenly
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