Microsoft says that its Outlook email services have been fully restored, following problems which lasted several hours.
Many Outlook users were unable to send email or access their accounts.
Hundreds from around Europe commented on the website Downdetector that they had been affected by the problem - many since Monday morning.
One common issue seemed to be that sent emails remained in the drafts folder and were not delivered to recipients.
On its website, Microsoft said the service dropped "unexpectedly" and it was working on a fix.
Not all account holders were affected.
"Intermittent connectivity is affecting customers in some European countries, which we are working to resolve as soon as possible," said a Microsoft representative.
Outlook incorporates Hotmail and Windows Live Hotmail accounts.
On its service status page, Microsoft said that an "alternate infrastructure" is being used while the service is restored.
"We've identified that a subset of infrastructure was unable to process requests as expected, which caused general service availability to drop unexpectedly," it says.
"We've redirected requests to alternate infrastructure to restore service, and we're monitoring the environment while connectivity recovers.
"Additionally, we're investigating an issue in which users are unable to send email messages."
In an updated statement, Microsoft now says services are "fully restored".