Customers of hosting firm 1&1 Internet have been plagued by email difficulties this week.
1&1 sells services such as web-hosting, domains, servers and email to many small UK businesses.
But this week customers have faced extended delays in receiving email, with one user complaining of an outage lasting since Monday.
It appears that two different problems have caused the disruption. 1&1 said the problem initially was caused by a surge in incoming email, causing delays. The ISP said this had been addressed by Thursday morning.
However, further technical problems mean the disruption has continued. "Following an important update to our mail server software and the subsequent restart of the mail servers, there have unfortunately been further delays in the delivery of incoming email," 1&1 explained. It claimed that as of Friday morning, the delay was two hours, but that disruption would continue throughout the day.
One user told ZDNet UK that she had been virtually without incoming email all week. Wendy McAuliffe, a director of Liberate Media, a public relations firm, said: "Our business relies heavily on email correspondence, and to have a drastically reduced email service for an entire week is totally unacceptable. To date we have received no explanation or apology. We have had to put alternative measures in place with no assistance from 1&1 Internet."
1&1 refused to say whether it would offer compensation to its customers, saying it was concentrating on reducing the delays. It said unhappy customers should contact its UK complaints department.
McAuliffe said that 1&1 had made no attempt to contact her and that its telephone lines were continually engaged. After finally making contact with the company on Friday morning, McAuliffe was told that 80 million emails were still queued.
1&1 told ZDNet UK on Friday afternoon that "several million" emails had been queued at the peak of the disruption, but denied that it reached 80 million.
No other 1&1 services are affected and the company says that no email has been lost.