Hotmail users around the world are reporting problems trying to login to their webmail accounts.
The problem were first reported on Friday, but seems to have escalated today. A number of users have written on the Microsoft Community website that they are receiving the following error message: "Something went wrong and we cannot log you in right now. Try again later".
It has been suggested that the problem might relate to users renaming their accounts to Outlook.com, as part of Microsoft's overhaul of the free webmail service.
Forum moderator Shikha_G responded to the issue on 12 October, claiming that Microsoft has "investigated the issue and fixed some things in the backend", so the error message should no longer appear.
However, some users claim that they are still seeing the same problem, and others are reporting getting different error messages such as "Sorry, there seems to be a problem with Hotmail right now," and "We're updating your inbox, please try again in a few minutes".
In some cases, these people have been unable to access their inboxes for over a week.
Others have logged into their accounts and been greeted with an apparently empty inbox, giving rise to concerns that their emails have been lost or deleted.
Techworld contacted Microsoft for a comment, but the company had not responded at the time of writing.
In the absence of an explanation from Microsoft, users have taken to Twitter to vent their frustrations.
"HOTMAIL FIX YOURSELF I HAVE AN ASSIGNMENT TO FINISH," wrote Twitter user @chelseacatilo.
"What the hell is happening to Hotmail?! Why has it gone all loony on my ass?!" said @PegasusZ.
"How can hotmail not have a customer service, blocking my emails for no reason," added @matinbehzad.
Back in August, Microsoft began rolling out a new webmail service for consumers called Outlook.com that will eventually replace Hotmail.
Microsoft hopes that Outlook.com will draw people away from competing consumer webmail services like Google's Gmail and Yahoo Mail.
The new service integrates with Exchange ActiveSync, so that Outlook.com accounts can be synchronised across a variety of devices. It also includes Office Web Apps, the online versions of Word, Excel, PowerPoint and OneNote, along with the SkyDrive cloud storage service.
Outlook.com is not to be confused with the Outlook email and calendaring PC application, nor with Outlook Web App, which gives Exchange users access to their accounts via a browser.