Companies could find it more difficult to protect their email systems from inappropriate content, following a recent court ruling in the US involving an ex-Intel employee.

The California Supreme Court last week ruled in favour of the worker, who emailed thousands of messages criticising the firm to corporate email accounts in 1996. The court ruled that the worker did not trespass on Intel systems with his actions.

The decision could make it more difficult for US companies to block unauthorised use of their systems in the future, as the onus would be on the firm to prove actual harm or damage to property, as opposed to property abuse.

Lawyers said companies in the UK could face a similar situation if trying to block a certain emailer.

"If the case did not amount to trespass under California law, I would be surprised if a similar set of circumstances would amount to an offence under the Computer Misuse Act in the UK," said Rupert Battcock, an IT lawyer at law firm Nabarro Nathanson.

Battcock advised companies to protect against this type of activity by putting into place a proper email policy, covering rights of monitoring and measures to manage liability.

"There is no reason why a policy should not also aim to deal with misuse by an ex-employee," he added.

Experts have also warned that firms could find themselves involved in legal action stemming from the deluge of pornographic spam landing in corporate inboxes.

This could lead to workers taking action against their employers for failing to protect them from inappropriate content. Cases of this kind are already beginning to appear in the US.

"If one company lost such a case, firms might have to take preventative action to protect them in the future," said Jamie Cowper, European channel manager at messaging system specialist Mirapoint.

"This might mean setting up a virtual private messaging network that only pre-approved senders could access."