Royal Mail in Gloucester has apologised for the letter service it has been providing, acknowledging it is "totally unacceptable".
Residents in the city have seen second class letters arriving late for months.
A public meeting was held between customers and representatives from Royal Mail in which they said they planned to change how the service works from September to "improve performance".
However, several people at the meeting, which was held in The Friendship Cafe in Gloucester on Monday, said they remained sceptical about whether or not the new system would work in practice.
Many of those questioning Royal Mail said they missed medical appointments or prescriptions because of postal delays.
Gloucester resident Wendy Worth said she missed an appointment to have a stent fitted because the letter from the hospital was delayed.
She only found out after a call from the NHS to say she had missed her appointment.
"Unfortunately I had to go private because I knew my blockage was quite severe," she said.
Nick Day and Alex Morrison, who were taking questions on behalf of Royal Mail at the meeting, said they would look into the getting medical letters to people more quickly.
Bob Whittard, who lives in the GL4 area of Gloucester, described the postal service there as "absolutely awful".
He said: "Letters [are often] four or five months late including credit card bills, inland revenue or stuff not arriving."
The Royal Mail representatives said that from next month second class deliveries would change to alternate days.
Day, who manages the mail primarily in Gloucestershire, said Royal Mail had already rolled out the plans in other areas, such as Coleford and Tetbury.
He said in Gloucester it was estimated that the backlog could be up to 25% in the sorting offices.
Losing staff
The company has been on a recruitment drive in Gloucester, employing 20 people since January but it has also lost 20 people in that time.
Additionally, some 30% of those coming into the company were leaving within 90 days, they said.
Morrison said Royal Mail had "some of the best terms and conditions for workers" but accepted that they needed to "do more to retain staff".
Royal Mail was working in difficult financial circumstances, Morrison said, having recorded losses of £336m in 2023/24 and £419m in 2022/23.
Despite the heavy losses the company recently returned a modest profit.
Gloucester MP Alex McIntyre set up the meeting.
He said he received more case work emails about postal delays then any other issue.
"We haven't seen an improvement yet," he said.
He said he hoped residents would start to see improvements from Royal Mail.
"I will be hauling them back in for another public meeting if things don't change," he said.
The Royal Mail representatives declined to be interviewed after the meeting.