The news comes despite Microsoft's recent acquisition of Sybari and GeCad and anti-spam company Giant, moves which many analysts thought would bring an end to its relationship with Symantec.
"Symantec Brightmail anti-spam is an important part of our overall spam fighting solution and we're pleased to extend out agreement with Symantec to help protect our customers from unwanted email," said Kevin Doerr, MSN Hotmail product unit manager at Microsoft.
Symantec was understandably buoyed by the extension of a deal first struck on November 2002. Shares in Symantec have risen markedly in the hours since its announcement.
Microsoft has long been a vocal and vociferous campaigner against spam. Earlier in the year SC reported a Microsoft lawsuit against Texan spammer Ryan Pitylak.
The news arrives on the same day San Francisco-based research group Ferris Research revealed that spam will cost US businesses $17 billion this year and that figure could grow to as much as $50 billion globally. The Global Economic Impact of Spam, 2005, highlights that most of this loss is due to a decrease in employee productivity.