The problem lies with one of Hotmail's servers that is undergoing maintenance, a Microsoft representative said without giving any more details. The access problem has affected fewer than 1 percent of the 67 million people who use Hotmail, the representative said.
"Hopefully, we'll be back up and running shortly," the representative said.
The representative did not know how many members have been unable to access their accounts or how long they have been locked out. But several members said they have been unable to access their accounts since last week.
Kevin Allen, information technology director for EMA Multimedia in Century City, Calif., said he has not been able to check his Hotmail account since Wednesday. Allen said he typically checks his Hotmail account three to four times a day and uses it to check his other email accounts.
"It's nothing that is huge," Allen said. But he added, "It is a little annoying."
Instead of being able to access their accounts, Allen and others affected by the maintenance problem have been receiving an apology message from Microsoft.
"Your account is temporarily unavailable," the note reads. "We do not expect this delay to last much longer, so please continue to check our site for your account status. We will do our best to make your account available as quickly as possible."
Microsoft shut down Hotmail for about an hour last month to combat a security breach. Last year, as the free email system expanded its membership, it suffered through a number of outages and service problems.