Residents across the South East say they are experiencing delays as they attempt to receive and send post in the build-up to Christmas.
Some people say they are only receiving one delivery a week and that they have been told to collect post from the sorting office instead.
One resident said there appeared to be a lack of staffing and if one postal worker was ill, there was "no cover".
Royal Mail said it was recruiting 16,000 staff for the festive period.
Diana Slack, who lives in Medway, said her husband had had a medical appointment which he only found out about because he was signed up to the NHS text service.
The letter arrived seven days after his appointment and had taken a total of 14 days to be delivered, she said.
She also did a name-a-star for her son, who had died earlier this year. This arrived after it was sent out three times and "it wasn't delivered by Royal Mail", she said.
Mrs Slack said: "I'll always question who will deliver my stuff in the future."
Newhaven councillor Sean Macleod said he has also been affected by the lack of postal deliveries in the area he represents.
He said people had been in touch with him, saying they had not received post for "weeks".
He added: "The sorting centre is chaos when you can go and collect mail. It's only open for a couple of hours and there's always parcels everywhere."
'Been ongoing for two months'
Richard, from Leybourne in Kent, told the BBC: "People are paying for a service they aren't getting. I subscribe to a weekly news magazine and the news I'm reading is a week old now.
"The issues have been ongoing for two months now. There seems to be no resilience in the staffing."
He added: "I probably won't be doing Christmas cards this year."
A Royal Mail van drives through a puddle
Royal Mail says it is hiring extra staff to cope with the extra demand over Christmas
Another resident in Ryarsh, near West Malling, described the lack of post she has been receiving as "worrying".Katherine said she nearly missed a medical appointment after her letter from the NHS was left in the sorting office in Snodland.She also did not receive her birthday card from her daughter until "over a week later", while a parcel she sent on 21 November only arrived at its destination on 4 December.Now, she said she is not prepared to risk Christmas and birthday cards not arriving, and will hand-deliver them to friends in Bromley - 25 miles away.She said the postal workers have told her they need to prioritise parcels.
'Recruiting more staff'
Royal Mail deny this, with a spokesperson saying: "We do not operate a policy of prioritising parcels. Ofcom recently reported, 'We did not identify any suggestion that Royal Mail senior management had directed the prioritisation of parcels over letters outside of recognised contingency plans.'"
They added: "Throughout Kent and Sussex, our teams are working extremely hard to ensure a consistent and reliable service to households this Christmas.
"We have increased our vehicle numbers and operational capacity, and we are in the process of recruiting 16,000 seasonal workers to help manage the increased demand.
"We can confirm we have mail being delivered to homes daily with the vast majority arriving on time, and if a delay occurs, we have dedicated teams in place to take immediate action."