The government says Royal Mail has given "an absolute commitment" to improve the postal service following hundreds of complaints about erratic deliveries across the east of England.
The BBC has been told that Royal Mail and its trade union are in "intensive talks" to try to solve the staffing problems, being blamed on an overtime ban and a new rota system being trialled at some delivery offices.
Dozens of customers in Milton Keynes, Hertfordshire, Bedfordshire, Cambridgeshire, Norfolk and Essex have been in touch with BBC's Politics East programme to complain about long delays in receiving their post.
Royal Mail insists that most post is being delivered on time and it has given the government "an absolute commitment" it will improve.
What is the reason?
According to the Communication Workers' Union (CWU) there are two main reasons for the current problems.
The first is a new shift system which is being trialled at 38 sorting offices around the country, including Milton Keynes, Southend and Ipswich.
It's called optimised delivery, which will see second class post delivered on alternate weekdays only on a two-week rotating schedule.
"We don't agree with it, it's putting too much strain on our people and it's clearly not working," said Mark Harper, CWU's regional officer for the East and the Midlands.
"We have a better idea about how this could be done and we have entered into intensive negotiations about this."
He said that at other depots an overtime ban meant when staff are sick they were not always being replaced.
"All our postmen and postwomen want to do is to get everything out in one day but at the moment they are being hamstrung because they aren't able to do that," said Harper.
Royal Mail said the changes were necessary because letter volumes had declined sharply from 20 billion a year a decade ago to 6.7 billion.
"We appreciate what Royal Mail is going through," said Harper. "It's not easy when people are sending fewer letters. It is a private company and it has to make a profit."
What customers are saying
Sara Chaves from Hitchin, Hertfordshire, was one of many people who contacted Politics East to complain about erratic deliveries.
"Last week we suddenly got a whole chunk of letters in one go and when we opened them we found a letter from the NHS that was dated 7th December and we realised that probably they had been accumulating letters for quite a while and delivering them all in one go.
"I opened a bank account at the beginning of January and I was supposed to return a letter to them within seven days, but because I got the letter way later I had to ask them to send another one."
"It is obviously frustrating.
"The point of a letter is to get a message and if you're not getting that message on time what's the point? We're going to have to do everything electronically."
Kevin Burdett, from Luton, said: "We're lucky if we get two deliveries within any given week.
"The problem was particularly highlighted last week when I received a Christmas card last Thursday from my cousin who only lives in Northamptonshire.
"He sent it way before the deadline for Christmas posting and yet it's taken a month to arrive at our doorstep.
"I don't think it's unrealistic to expect a better level of service. A first class stamp costs £1.70 these days and certainly the cost does not correlate with the level of service we receive.
"My message to Royal Mail would be 'you've got to do a lot better gentlemen and ladies'."
What is supposed to happen?
Under its universal service obligation, Royal Mail is expected to deliver to UK addresses on every day of the week except Sunday.
It warns very remote or hard-to-reach locations might see slightly longer delivery times, especially during bad weather.
Ofcom, which oversees Royal Mail, has set a target of 93% of first class letters being delivered within one working day and 98.5% of second class post within three working days.
However, it fined the company £21m after both targets were missed.
Between April 2024 and March 2025, just 77% of first class mail was delivered on time and 92.5% of second class mail.
What does Royal Mail say?
Royal Mail says it understands customers' frustrations and is keen to stress that "the vast majority" of mail is delivered as planned.
In a statement, it told us: "Where a delay affects a route, we put in extra support and review performance daily to restore regular deliveries as quickly as possible.
"While we know some customers have experienced delays, reports of waits lasting a month are not representative of delivery performance across the area."
What is the government doing about it?
The post office minister told MPs this week he had met with Royal Mail and "was given an absolute commitment that they will work to deliver the best possible service to customers".
"I've made it clear that people not getting their mail is simply not good enough. Customers, particularly those in rural areas, must see visible sustained improvement in reliability, timeliness and delivery office performance."