Less than five percent of all email is delivered to mailboxes, as the rest is junk blocked by spam-fighting efforts, according to Enisa, the European Network and Information Security Agency.
The agency reported the finding in a report, published on Thursday, on anti-spam measures taken by European ISPs in 2009. While these measures are well used by providers, junk email remains a key problem for them and takes up a large part of their annual budgets, according to the report.
ISPs surveyed by the agency said nearly 80 percent of all SMTP connections were aborted, mainly because a match had occurred against a blacklist. Of the connections that were accepted, almost 80 percent were filtered out, mostly as spam, it added in its report released on Thursday.
The result is that only 4.4 percent of all email was delivered, down from six percent in Enisa's last spam report two years ago, the agency said.
"The data on aborted SMTP connections and filtered emails seems to show that anti-spam measures are currently highly effective," Enisa said in its study.
Blacklists are the most commonly used technique for preventing the sending of spam, the agency found, followed by the placing of limitations on high volumes of outbound mail. Both techniques are used by more than 60 percent of ISPs.
However, false positives emerged as a problem with blacklisting. Two-thirds of companies, including all the largest providers, said they had had servers incorrectly added to or retained on lists of offenders.
"With blacklists so important in blocking spam, their reliability is crucial," Enisa said in its study. "This high level of responses citing problems with blacklists incorrectly including non-spamming servers is alarming."
Enisa said ISPs had made few major changes in their efforts to combat spam since its last survey, in 2007.
"Most measures are applied by similar proportions of providers to what was observed in 2007. Usage of the main types of sender-authentication mechanisms remains approximately the same. Abuse report handling is still mostly manual. And the percentage of respondents perceiving conflicts between spam filtering and ISP obligations has remained steady," it said in its report.
Nevertheless, some 70 percent of ISPs said spam was a 'highly significant' part of security, and this importance was reflected in their budgets. One-quarter of very small providers said they spent more than €10,000 (£8,700) per year on fighting spam, and one-third of very large providers invested more than €1m per year.
The agency noted that many, though not all, providers currently use collaborative measures to fight junk mail, such as working with spam-sending ISPs to eliminate the problem. It recommended that more service providers should work together on the problem.
For its report, Enisa surveyed 100 ISPs from 30 countries, including 26 of the 27 EU member states. As a further step towards addressing the root causes of spam, the agency said it plans to deliver a report on botnets by the end of 2010.