People living in blocks of flats have not received any post since November because of a row over an entry fob.
It has led to missed hospital appointments, and online orders not being delivered as thousands of items of post stack up in the sorting office.
The issue affects residents living in flats managed by social housing provider Torus in Brainerd Street, Old Swan.
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Royal Mail said they cannot gain access to the property without an entry fob.
It is thought Torus has been asked for one of these items for a number of months, including escalating it to head office.
A key fob provided by Torus on January 12 reportedly did not work, and Torus has been informed of this.
In the meantime, residents are left without any post reaching their homes. One, Janet McKinney, said she had reached the point where she had offered to pay for a fob herself.
Janet, 63, said: "We've not had any post for over three months now and nobody seems to be doing anything about it.
"They now have bags and bags of post which have built up at the sorting office. Even if we go down there in person to try and get something, they won't give us any because there's now too much lying around for them to search through.
"There are literally hundreds of people affected by it. It's all down to how the delivery workers get access to the flats. They just need a fob to get in to deliver our mail, but Torus won't give them one and say they should use a trade button instead.
"However this trade button is only answered between the hours of 9 and 11am, which isn't when the postal deliveries are generally made."
Janet added: "It's now got so bad that I've offered to pay for a fob myself.
"In November it was my birthday, and none of the cards sent from my family in Ireland arrived, and my husband has also missed hospital appointments.
"Others have ordered stuff online which hasn't arrived. Everybody is absolutely up the wall. It's just been a nightmare."
A Royal Mail spokesperson said: “We apologise for the inconvenience caused to our customers by the problems that we have experienced delivering mail to properties on Brainerd Street.
"To deliver to these properties, we need to be provided with an entry fob by Torus Group, and we are urgently pursuing this with Torus. Once we have received the correct entry fob from Torus, we look forward to resuming deliveries, and we hope that this will take place as soon as possible."
A spokesperson for Torus said: "We are aware that an issue with access at Brainerd Street has disrupted Royal Mail postal delivery and issued new key fobs to Royal Mail on January 12 to rectify this issue.
"However, we are aware that there are still ongoing issues and are investigating these further. We will continue to work with Royal Mail over the coming days to ensure residents receive their post as soon as possible and apologise for any inconvenience caused."
