Website Downrightnow.com, which monitors popular online services, displayed a graph showing multiple disruptions of Yahoo Mail in the past 24 hours.
"Yahoo mail is down... AGAIN," one Twitter user wrote, using the hashtag #yahoomail. "Totally frustrated."
"Anyone else having constant problems with @yahoomail lately? After 15 years, I'm on the edge of abandoning it #yahoomaildown," said another.
In an email response to an AFP inquiry, the California-based Internet firm said it was "working to mitigate a Yahoo Mail issue that is limited to a small segment of users."
"We apologize for the inconvenience as we certainly understand email is a critical service for our customers," it said.
The service was almost back to full functioning as of early afternoon in California, according to Yahoo.
Yahoo said its email service has 110 million active daily users.
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