"In the ongoing efforts to continue to improve the Hotmail service, as part of that process, MSN services come on- and offline periodically," a Microsoft representative said. "With that there have been MSN Hotmail users who have periodically been locked out of their Hotmail account."
Microsoft says it has 84 million Hotmail members. "A limited number have been impacted," and the upgrade is "nearly complete," according to the representative.
CNET News.com began receiving complaints from members early this week who said they have been periodically locked out of their accounts since as early as Dec. 5.
"I use Hotmail for backup email and when I'm on a job hunt, but I have thought of using paid service email because reliability is extremely important, and today's Hotmail problems have encouraged that possibility," wrote Hotmail customer Jamiel Humayun.
This is not the first time Hotmail users have confronted problems with the free email service. The company suffered a similar outage almost a year ago when it failed to pay a $35 registration fee for the domain name Passport.com.
Passport.com, the authentication service for Hotmail, verifies usernames and passwords. This problem was fixed when a Linux programmer noticed the problem and paid the $35 registration fee for the domain name. Microsoft later reimbursed him.
Other free, Web-based email services also have been plagued by problems. All this comes with the territory of a free account, according to Jupiter Media Metrix analyst Christopher Todd.
"Consumers can be frustrated with it, but the fact of the matter is they're not paying anything for it," he added.