Pump-and-dump spam, which dominated inboxes in the first half of the year, has given way to image spam and messages pushing 'sexual enhancement' drugs, according to a report from security firm BitDefender.

In the seven months since January, BitDefender found that 75 per cent of all image spam was penny-stock related, while text-based spam was the preferred choice for drug-related topics.

Spam messages for sexual enhancement and weight loss drugs accounted for 56 per cent of all text spam. This was followed by spam hawking replica watches, which came third in BitDefender's image and text spam lists.

New types of spam have also emerged in 2007, most notably hosted-image spam, which instead of containing an image provides a link to a website hosting the spam.

Spam with attachments, such as a PDF file, has also become more prevalent since the beginning of 2007.

The latest measures used by spammers to try to defeat filters are malformed mail boundaries, which make it hard to unpack emails for inspection, and the use of malformed HTML code in an attempt to confuse parsers.

So-called Bayes poisoning and word obfuscation were used less frequently than in previous years, according to BitDefender.

Vlad Valceanu, head of BitDefender's Antispam Laboratory, said: "Spammers are emailing fewer targets at a time, while introducing small variations in every email in the hope of avoiding timely detection.

"We expect to see more of this in the next half of the year, as well as increased use of attachments and possibly even embedded Flash."