The Royal Mail app and website was unavailable on Wednesday for more than four hours, users said.
The outage comes on the first day of regular post following strikes and bank holidays over the Christmas period.
Down Detector, which tracks websites, showed that thousands of people had reported that the Royal Mail site was inaccessible.
Visitors were presented with the message: “Our website is temporarily unavailable.”
At the time of writing, some users continue to report they remain unable to access the Royal Mail app or website but reports on Down Detector are declining.
As well as the reports in the morning of 28 December, there were hundreds more made overnight, beginning at 9pm on 27 December.
A Royal Mail spokesperson told the BBC: “We are very sorry for the difficulties experienced by some customers when using our website today.
“We are working to resolve this technical issue as quickly as possible and apologise for any inconvenience caused.”
News Source: BBC