Security experts have warned that a high-level outbreak of Trojan horse programs is hitting networks around the world.
Email security company MessageLabs said it has blocked 54,000 copies of new Downloader Trojans since 02:00(BST) on Thursday.
Alex Shipp, senior antivirus technologist for MessageLabs, said: "They are pretty run of the mill — they use email subjects that have been used before. But we're detecting them from all over the place."
The emails, which contain the Trojan horse programs Downloader.ABC and .XZ, purport to be a spam report from an IT administrator. The messages request people run an attached file to prevent their email account from being shut down. MessageLabs said people could be fooled into opening the attachments, which — when run — open a backdoor on the infected computer for hackers to use.
MessageLabs said in a statement both Trojans have the same email characteristics and rated the outbreak level as "high".
The message carrying the Trojans reads:
"Your e-mail account was used to send a huge amount of unsolicited spam messages during the recent week. If you could please take 5-10 minutes out of your online experience and confirm the attached document so you will not run into any future problems with the online service.
"If you choose to ignore our request, you leave us no choice but to cancel your membership.
"Virtually yours, Network Administrator Team"