The e-mail security provider found that of all the e-mail containing attachments scanned by the firm between March and August this year, 0.02% contained inappropriate or potentially sensitive images such as pornography, cartoons, jokes, greeting cards and other non-work related material.
This compared to 0.07% for the same period in 2003.
"While we cannot say for certain what has caused this drop, one possible explanation is growing enforcement of corporate governance requirements," said Mark Sunner, Chief Technology Officer at MessageLabs.
"For companies that use e-mail to conduct critical business transactions and communications, corporate governance can be a great concern. We are now seeing a number of organisations using e-mail management solutions to help ensure compliance and reduce risk. The effect of this could be one of the reasons why fewer inappropriate images are being sent via e-mail."
MessageLabs also found a drop in the number of spam and viruses circulating last month.
Of all e-mail scanned by MessageLabs' Anti-Spam service, 84.2% was categorised as spam, compared to 94.5% during July 2004. And 6.9% of the e-mails scanned by MessageLabs' Anti-Virus service were identified as virus-infected during August, a decrease from the 7.3% intercepted during July.
Sunner put the drop down to the season, explaining that the summer is often a quiet time for virus and spam distribution. He also suggested that the figures might have been affected by Operation Web Snare – a US initiative that has led to the recent arrests of 150 individuals.
"This operation may have removed some of the perpetrators from active service and could have worried people enough to suspend operations for now," said Sunner.