Spam levels have risen over the past month to more than 90 percent of all corporate e-mail, according to Symantec’s May 2009 MessageLabs Intelligence Report (PDF). The latest report effectively communicates the concept of "spam, boy there sure is a lot of it," but goes into detail about the latest trends in spamming activity like botnet activity and the use of social networks.

In May, spam rose by 5.1 percent over April, with 57.6 of it coming from known botnets. One particular botnet called Donbot was named as the most active, and is responsible for 18.2 percent of all spam. Symantec wrote that much of the remainder (42.4 percent) of spam originated out of smaller or unclassified botnets.

Despite the seemingly automated botnet activity, spammers are apparently most active during the US working day. As Symantec noted in the report, this could be indicative that most active spammers are based in the US, or that they find the US workforce to be the best targets. "Spammers are finding this large target audience that’s online and more likely to respond as being very profitable for their nefarious activities," the company wrote. Spam levels also "drop significantly" on Sundays in all regions.

Symantec also pointed out that, according to its research, "[t]he common assumption that most web-based malware resides on less reputable websites, perhaps touting adult content, was called into question." Instead, the company found that malicious content is often hidden on well-established domains, particularly social networking sites or those that rely heavily on user-generated content.

The report also made note that image spam seems to be making a comeback, corroborating a report from IBM's X-Force team earlier this month that image spam was back on the rise after hitting record lows last year. IBM wasn't sure why spammers decided to resurrect this technique, but image spam skyrocketed to 22 percent of all spam in April.

Of course, there's some disagreement on exactly what percentage of e-mail is spam—Microsoft claimed earlier this year that it was 97 percent. But we know one thing: spam-wielding botnets will keep growing as as filtering technology continues to mature. Hey, somebody's gotta make up for that drop in clickthroughs.