AOL has fixed a major problem that affected its tens of millions of email users worldwide for several hours last week, but what caused the snarl remains a mystery to the internet access and services provider.

A software glitch tripped AOL's email system causing delays in the sending and delivery of messages for about five hours, according to an AOL spokesman.

The snag affected both paying subscribers and free webmail users from a variety of AOL services, including the flagship AOL service, CompuServe and Netscape, as well as the mail service provided to users of the AIM instant messaging network.

All messages were eventually delivered successfully, the spokesman said via email. "There is no longer any member impact on the AOL service from this rare and isolated incident. We will continue to investigate its cause," he wrote on Friday.

Email is a key offering for internet companies such as AOL, Yahoo and MSN, because it ties users tightly to their network of online services and beefs up the audience these vendors promise to advertisers. Precisely because users form such strong attachment to their email accounts, any downtime is sorely resented, and even more so when the problem is prolonged and widespread.

In the past two years, most major providers of online services, including Yahoo and MSN, have invested in upgrading the functionality and capacity of their email systems, particularly after Google launched its Gmail service in 2004, which rocked this market with its ease of use and, at the time, unparalleled free storage capacity.

AOL is in no position to risk disappointing its users, particularly its paying subscribers, who in the past several years have been deserting the service in droves. AOL is also beefing up its free websites, such as, to significantly increase usage and consequently the revenue it generates from online ads.