Users of Microsoft's web-based email services have been hit by another round of technical difficulties, as the firm's Hotmail to Outlook.com upgrades gather pace.
According to the software giant's Service Status page, Hotmail and Outlook users have experienced email access problems since yesterday evening.
The firm's cloud-based storage service SkyDrive also experienced service troubles that made it difficult for users to add, edit and remove files, but these were fixed in the earlier hours of this morning.
"You might not be able to see all your email messages. We're working to restore service right now," said the company in a Hotmail service update.
"Fixing the problem is taking longer than we hoped...We apologise for the lengthy interruption in service," it concluded.
The downtime reports are appearing on Microsoft's main Service Status page, suggesting this is a wider-scale outage than the one that blighted Hotmail users last month.
At the time of that outage, Hotmail users complained that Microsoft had not kept them updated about when the service would be resolved.
In a statement to IT Pro at the time, Microsoft said, because of the small percentage of users affected by the outage, affected users would have seen a personalised status update when they logged into their accounts.
The migration of Hotmail users to Outlook.com is expected to be completed in the summer, but has been beset with issues so far.
As reported by IT Pro last week, many users claim to have been updated without being notified first and have complained about the system's new Windows 8-like user interface.
Updated: Microsoft has since claimed yesterday's Hotmail and Outlook service problems were caused by an unexpected temperature spike in one of the firm's datacentres.
In a blog post, the company said the temperature rose in a "rapid and substantial" way following a firmware update at a core part of the physical plant.
"This is an update that had been done successfully previously, but failed in this specific instance in an unexpected way," the post stated.
"This [temperature] spike was significant enough before it was mitigated that it caused safeguards to come into place for a large number of servers in this part of the datacentre.
"These safeguards prevented access to mailboxes houses on these servers and also prevented any other pieces of our infrastructure to automatically failover and allow continued access," it added.
This article was originally published on 13 March, but updated to include details of Microsoft's response.
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