Subscribers to the Microsoft-owned Hotmail web-based email operation are collectively receiving more than one billion pieces of junk email daily.
Parul Shah, product manager at Microsoft, admitted that spam accounts for 80 per cent of messages that pass through the company's servers.
Shah said the figures did not include spam that was automatically detected and rejected by Hotmail's own filtering system.
He said that although Hotmail has developed various tools for evading spam - including user-driven filters - large amounts still get through, with the percentage of spam slipping though the filters increasing.
Shah admitted that the task of keeping junkmail out of a server is nearly impossible as spammers always work out ways of getting around systems.
"Every time Hotmail or another email service provider finds a way to detect spam, the spammer immediately has a way to get around that," he said.
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