The volume of pornographic spam almost trebled during May, according to newly published research.

Security vendor Clearswift reported in its monthly email analysis that spam relating to pornography rose from 5.6 per cent to over 14 per cent of total spam sent in May. Healthcare spam remains the dominant form at 44 per cent.

"Perhaps the sickest spam over the last month cashed in on the blanket media coverage of Michael Jackson's trial," the report said.

"Entitled 'News from Neverland Ranch' the email claimed that Jackson, since acquitted, had attempted suicide in advance of his trial.

"Clever social engineering meant that many email users, intrigued and shocked by the news, clicked on the link enclosed in the email and downloaded a Trojan which secretly installed malicious software onto their PC."

Clearswift also warned that the spamming of electronic greetings cards is again emerging as a problem. It noted that this technique is particularly effective at generating click-throughs from spam since the emails purport to be from friends.