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The amount of junk e-mail being sent across the globe has seen a dramatic fall in recent months.

The volume of spam has dropped steadily since August, but the Christmas period saw a .

One security firm detected around 200 billion spam messages being sent each day in August, but just 50 billion in December.

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 ����ý News.

"But it's hard to say why," he added.

Inactive botnets

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.

In December, Rustock was responsible for just 0.5% of global spam, he said.

At the same time, two other prominent spamming botnets, Lethic and Xarvester, also went quiet.

There have been huge drops in spam levels before, said Mr Wood.

"Usually they have been associated with the botnets being disrupted. As far as we can tell Rustock is still intact," he added.

That means those controlling Rustock could have continued churning out masses of spam, but for whatever reason, have chosen not to.

Chasing profit

One possible explanation is that the spammers are simply regrouping ahead of a new campaign.

Spammers are driven entirely by profit, said Carl Leonard, a researcher at security firm Websense.

"So if a campaign is not getting the returns they want, they can stop, regroup and try something else," he said.

Anti-spam campaigns have enjoyed recent success in making life difficult for spammers, said Mr Wood.

In late September 2010, a collective known as Spamit announced it was closing because of "numerous negative events" and increased attention.

That has certainly contributed to the current decline in spam volumes, said Vincent Hanna, an investigator at anti-spam group Spamhaus.

"This was a significant operation, with assets all over the world. Its decision to stop operating - or at least lay low for a while - has made it more difficult for [other] spammers," he said.

That helps explain the longer-term drop, but the reason for the reduction in December in not yet understood, he added.

There have, however, been signs that spammers are turning to alternative methods to e-mail for distributing their messages - such as Facebook and Twitter, said Mr Leonard.

In December, Twitter accounts were hijacked to distribute diet pill spam after a list of possible passwords was published online.

Even so, it is still too early to say the current lull in activity will last, said Mr Leonard.

"For years there have been predictions that e-mail spam is set to decline," said Mr Leonard. "But for as long the spammers can generate profit from their activities, it's not going away."

Mr Wood said new spammers usually pop up to replace inactive ones.

"We've yet to see any evidence that spam has become a bad business to be in," he added.