Anger at EE's Orange webmail problems
EE's Orange webmail has been unavailable for many users for about a day, after the service suffered problems starting on Tuesday evening.
The firm said a fibre-optic cable had been cut and could not say when it would be fixed.
An EE spokeswoman said a small proportion of the 30 million users of its network had been affected.
Some customers expressed dissatisfaction at what they said was a lack of communication from the firm.
One BBC viewer complained that EE had not posted an explanation on its website. John Edwards said he and his wife were worried that they would miss an event because they could not print tickets held in their inbox.
"You have got holiday tickets, train tickets, theatre tickets, tennis tickets - you name it. The system today with a lot of these suppliers is to do it by email. People cannot get their tickets," he told the BBC.
EE has engaged with some affected users on Twitter and posted a statement on the website-monitoring service Down Detector.
"A lot of people are complaining bitterly about the lack of information or the fact that the system has crashed.
"Why do they not make a public announcement and be quite open about [it]?" asked Mr Edwards. "That could be done through Twitter but I am not a Twitter user, I am not a Facebook user."
EE also fielded complaints from customers on Twitter.
In its statement to customers, EE said: "We have now identified a technical problem with a third-party fibre-optic cable that leads into one of our major data-centres.
"We understand the cable has been cut and we're in the process of replacing it. Needless to say we have a number of teams working to fix it as soon as possible.
"We are really sorry and know how frustrating outages such as this can be. We'll let you know as soon as the fix is in place."
A spokeswoman added that only those customers trying to access their email through a web browser, rather than through an app, were affected.
She could not say how many users had been affected, but said she believed it to be small proportion of EE's total customer-base.