(CNN) -- A days-long outage for some Yahoo e-mail users is casting a dark shadow over a perceived turnaround at the once-mighty Internet player.
Some users of Yahoo Mail, which underwent a major overhaul over the summer, were still without service Thursday from an outage that started late Monday.
The downtime was due to "a hardware problem in our of our mail data centers" and proved "harder to fix than we originally expected," Jeff Bonforte, who heads up e-mail services for Yahoo, said in a blog post.
Late Wednesday, the company posted that the affected servers were up and running, but that it would take time to catch up on missed e-mails and that some users would still have trouble accessing their accounts.
The post said it expected all lagged e-mails to be delivered by Thursday afternoon.
CEO Marissa Mayer made revamping e-mail a part of her effort to kickstart a Web company that, in the eyes of many, had grown stale and was bleeding money.
It's been a rocky process.
Announced last December, the first e-mail overhaul rolled out to all users (whether they wanted it or not) in June. So lukewarm was the reception that it got a largely cosmetic redesign again in October. The result? More outrage.
Under Mayer, Yahoo purchased Bonforte's company, Xobni, in July, and the former Yahoo staffer was put in charge of the redesign. But it's suffered from periodic outages since October, the latest being the most dramatic.
Kara Swisher, of tech blog All Things Digital, wrote a scathing piece tackling recurring problems with Yahoo Mail and the company's response.
"While I am certain Bonforte -- for whom I have had great respect for a long time -- and his team are trying their best to fix the problem, the contrast to Yahoo CEO Marissa Mayer's insistence on engineering excellence and innovation and the inability of the company to provide a cogent explanation about what is happening to one of its most important daily interfaces with its customers could not be any more stark," she wrote.
"In other words: What could be bigger than a Fail Whale? This."
Rank-and-file users were no happier, based on social media posts about the outage.
"Update 10 hours old, account still not accessible by phone. Your 'help' said to reinstall it, but account not available for that," one Twitter user wrote to Yahoo's mail team account Thursday morning.
"A freakin nightmare; you guys work for Govt healthcare?" wrote another.
Since Mayer took over in June 2012, there have been signs of life at the company. Stock prices have jumped 130% as she orchestrated a logo redesign, several high-profile acquisitions -- most notably Tumblr -- and overhauls of apps, including e-mail.
But Google and Facebook continue to chip away at Yahoo's biggest moneymaker, online advertising, and revenue has been flat.
A black eye for something as simple as an e-mail outage, and a response viewed as lackluster by some, is the last thing a company on the rebound wants or needs.
"The issue isn't that Yahoo's webmail has gone down — in fact, from time to time it is to be expected," Lauren Hockenson wrote for tech blog GigaOM. "But when one of a company's core services, especially the one that CEO Marissa Meyer identified as the linchpin to the success of Yahoo, experiences issues, users expect to be informed regularly. The company must be transparent about outages, or risk ruining the reputation it has worked so hard to rebuild."