Pre-Christmas Royal Mail delays throughout Bolton led residents to miss important appointments, according to MP Phil Brickell.
In some cases these delays led certain Horwich residents to miss critically important NHS appointment letters.
The Royal Mail responded to to Mr Brickell's letter, though he believes their answer to his question was 'inadequate' and will be pursuing the matter further.
In his letter, Mr Brickell said: "Having visited the sorting office last year following the Christmas period, it is clear that the space is no longer suitable to deal with the increase of parcels over the past few years.
"In addition, it appears to me that the ability to buy a first-class stamp during the festive period is completely redundant, given that there appears to be no prioritisation of letters.
"NHS letters are being delayed during this period which can cause a huge amount of do stress for my constituents.
"Where these letters are not delivered to those who do not have the NHS app, this can lead to missed appointments and a knock-on effect for other patients.
"Following a meeting with the Business Minister, Blair MacDougall MP, it is clear that these issues of capacity and delivery of service are widespread with a number of my MP colleagues sharing almost identical experiences with the minister."
Mr Brickell urged the Royal Mail to implement plans to ensure the same delays are not repeated in 2026, asked to know whether more staff would be hired, and asked whether NHS letters are being prioritised 'as per the government and royal mail agreement'.
Mr Brickell's correspondence with the Royal Mail comes at the same as news of 'potential service delays across more than 100 UK postcodes' caused by 'storms and workers being off sick', according to PA Media.
In response to Mr Brickell's enquiries, a Royal Mail spokesperson said: “We understand how frustrating it can be when post does not arrive as expected.
"The vast majority of addresses in Bolton are receiving mail regularly, and we are working hard to restore consistent deliveries wherever disruption has occurred.
"Where this is not yet possible, deliveries are being made at least every other day while we work to return to normal service levels.
“We are committed to delivering both letters and parcels reliably and are taking the necessary steps to strengthen service for customers in Bolton.”
Royal Mail has also been asked to respond to a group of MPs who raised concerns about “chaos” in the postal service since Christmas and suggestions that some letters are being delivered in “batches”.
The company has been given two weeks to respond to a series of questions put to it by the Business and Trade Committee (BTC).