According to Campaign Monitor, the incursion allowed hackers to gain access to customer accounts, which they cracked by importing their own lists of harvested emails in order to launch spam campaigns.

Because the harvested emails were using `clean' server IDs, the spam stood a much greater chance of reaching its destination, Infosecurity notes.

The good news is that Campaign Monitor said no credit card details of customers was involved in the hacker attack and consequent network incursion.

The firm said the brunt of the hacker attack took place late on Saturday and early on Sunday, although attack effects lasted into Monday.

In a posting, Campaign Monitor said that the company has been gathering information so that it can contact users with accurate details, and also making sure it was stopping ongoing problems.

"We did not want to give you incomplete or misleading information. Right now we are still finding out more, but it is important you are all aware of the situation," the site told its customers.

"We are still actively working to get full detail on this, but essentially one of our servers was compromised, and that gave the hacker enough access to be able to get into a few customer accounts. We now know more, but don't want to publish any details as you can understand," it added.

According to newswire reports, Campaign Monitor has notified the owners of the affected accounts, and has commissioned an external IT security audit to be carried out.