Vital letters including hospital appointment notifications, bank cards and work documents have been missed because of postal delays, according to residents in a South Yorkshire neighbourhood.
People living in Nether Edge and Sharrow said letters had been delayed since Christmas - but they had continued to receive parcels and items sent by recorded delivery.
Royal Mail said the "temporary build up" was due to extreme weather and high sickness rates in a local sorting office.
However, Ibby Ullah, councillor for Nether Edge and Sharrow, said the company's communication had been "utterly poor".
Ullah said he was originally notified of the issue by a handful of residents but soon found out it was "a more widespread issue across the ward and in other parts of Sheffield".
"In the UK we still are reliant on the postal system to receive formal letters. One resident said they've had to pay a fine because they didn't pay a tax bill on time, after having missed the letter," he said.
Charlie Webster, who lives in Nether Edge, said she noticed the problem in mid-January.
"Items sent at the start of the month, including cheques, didn't arrive. It soon became clear there was a serious backlog," she said.
"It was obvious this wasn't just us. Most of the street was missing post."
Another resident, who asked not to be named, said he had not received any post for 28 days.
"I had hospital letters in there which I didn't even know about. I got a message saying 'your appointment's been cancelled, you hadn't turned up'," he said.
"The worst thing about this is, you ring up (Royal Mail), nothing happens. They're not interested."
Ken Smith, who is chair of the Peak District National Park Authority, said he had missed "important work documents" sent by post to his home in Nether Edge.
"We are aware that post is taking longer to be delivered across the country and it impacts on people, it impacts on businesses, but this was the first example I'd had where lack of material arriving in time for meetings was beginning to impact on my ability to do my job," he said.
He said his workplace had complained to Royal Mail last year about mail not being delivered, including about one item that never turned up.
He added that the backlog appeared to have been cleared now but said, "there's no guarantee it won't happen again".
Ullah and Sheffield Central MP Abtisam Mohamed said they were now seeking a meeting with senior figures at Royal Mail, "to get reassurance that this won't happen again".
A Royal Mail spokesperson said: "Severe snowfall and ice at the start of January caused short-term disruption at Sheffield South delivery office.
"During this period, we prioritised the safety of our staff and were unable to deliver to addresses.
"This created a temporary build-up of mail, which was compounded by high sickness absence."
They said action was taken to "clear delayed items" and "at no point has there been four weeks' worth of mail in the office".
The spokesperson confirmed there was a reduction in staff overtime after Christmas but added that this was because the volume of work decreased compared to the peak period.
"However, we continue to review performance daily and use overtime where needed, and it continues to be used in Sheffield," they said.