Yahoo (NASDAQ:YHOO) CEO Marissa Mayer tweeted yesterday that Yahoo Mail had been "restored" and its service outage was over, but customers were still complaining on Twitter about log-in issues on Thursday. Yahoo Mail has been down in an outage that lasted over three days, and is struggling to begin delivering users' messages again.

Many customers claim that their that attempts to log in to Yahoo Mail were thwarted by notifications – that the service was undergoing “maintenance.”

Yahoo sent an email late on Wednesday to tell users it had restored POP access to the service, and stated that 70 percent of messages sent during the outage were not yet delivered. Yahoo said undelivered mail should arrive in users' inboxes sometime Thursday afternoon, but it was still working to deliver other affected emails -- sent between Nov. 25 and Dec. 9 -- to users.

That means Yahoo Mail had not yet delivered emails that were sent to or from its service since three days before Thanksgiving.

Yahoo tweeted on Dec. 3 that a “small (percentage) of our users are seeing intermittent delays receiving emails (and) accessing archived emails. We are working to resolve ASAP.” It is unclear if this earlier issue was ever resolved, or permitted to spread and eventually affect a larger number of Yahoo Mail users.

Yahoo Mail Outage Down CEO President Marissa Mayer
Yahoo CEO Marissa Mayer Reuters

The Yahoo Mail team tweeted again on Dec. 10 that it knew “some of our users are unable to use Yahoo Mail. We’re on it & trying to restore access in a few hours.” However, the team has not divulged what caused the issue, and as of this writing has not returned a request for comment.

Two days and 10 tweets later, the Yahoo Mail team updated a page on the service’s status with the following:

Here’s our latest update: We can confirm that 97% of affected users have access to their Mail accounts on web, POP and the Yahoo Mail iOS, Android and Windows 8 apps. For these users, we have delivered 80% of their queued messages that were sent from 10:27 PM PT on 12/9 until now.

We're aware that some users are still having trouble accessing their accounts. We're working tirelessly to restore access to their accounts.

We would like to clarify that for users impacted by this outage, their Yahoo Mail was working normally from 11/25-12/9. We're still working on restoring messages that were delivered during this time.

We're slowly ramping up IMAP access to ensure stability and maintain current users’ access.

It appears that Yahoo Mail users who access the service with a non-Yahoo app, such as Microsoft Outlook or Apple Mail, had the most issues. The problems, Yahoo Mail says, were due to an issue sending and receiving IMAP & POP messages.

The company has not commented on those unable to sign in to Yahoo Mail through a desktop browser. Several unhappy customers began directing tweets at Yahoo, using the hashtag #yahoomailfail.

Many users said they would be switching to rival service, Gmail. Google (NASDAQ:GOOG) announced today that it would begin routing Gmail images through its servers and no longer force users to click “show images” in every email containing them. Instead, Google servers will transcode images which it says will protect Gmail users from malware.

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