Thousands of letters are delivered to the wrong address every month but few people take the time to complain to Royal Mail, a consumer group has said.
Some people admit to opening wrongly delivered letters
Postwatch estimates 14.4m letters are lost every year - and of these 60% are just put through the wrong letter box.
The watchdog has urged people to tell Royal Mail about mistakes, saying it causes problems for people and firms.
A spokesman for Royal Mail said they had "substantially cut" the number of items mis-delivered, delayed or lost.
Postwatch said 2,000 people had made complaints about mis-delivered post in the past year.
But a survey by the group of 2,100 people found more than half had received mail not intended for them in the past six months.
About half delivered it themselves to the right address but one in 20 said they threw the item away and a handful admitted they had opened post which was not theirs.
Postwatch said only 10% of customers complained to Royal Mail - and even those who posted the letter again or delivered it themselves took up to a week to do so.
The group is publishing a pack urging customers to let Royal Mail know about mistakes.
Postwatch chairman Peter Carr said: "Our message to customers is that if Royal Mail do not know about a problem they cannot fix it."
He said the group had evidence of residents in Newtownabbey in Northern Ireland missing hospital appointments or failing to receive benefits on time because of wrong deliveries.
An adventure holiday firm in Bangor, north Wales, said lost post had cost it thousands of pounds in business.
LETTER BOX LITANY
Postwatch claims 12.4m letters are lost each year
Of these, 60% are delivered to the wrong house
One in 10 people complain to Royal Mail
One in 20 people surveyed said they threw post away
People take up to a week to post on letters
A handful of people open post not meant for them
Royal Mail says 322m items were undeliverable in 2002/3
About 12m items a year are posted with only the first name or surname
The amount of undeliverable mail has gone up by 50% in five years
"It may not seem important if you get someone else's mail," Mr Carr said.
"But thousands of letters every month are accidentally put through the wrong letter box and are never passed on to the correct address and that letter sitting in your home could mean a great deal to the person it is addressed to."
The Communications Workers Union blamed underinvestment in the service and training for delivery mistakes.
Deputy general secretary Dave Ward said Royal Mail should stop using too many agency staff, improve sorting and delivery training and employ more full-time workers on a higher salary.
"These steps, the union believes, would practically end the problem," he said.
The Royal Mail spokesman said: "We have been working hard to improve our services and have substantially cut the amount of mail that is mis-delivered, lost or delayed."
He said the latest figures show the amount of mail lost and delayed had halved in 2002/3 from the previous year's figure.
About 280,000 letters a week were lost or substantially delayed in 2002/3 - about 0.07% of the total.
He said: "Every single letter matters to us, so we are continuing to all we can to reduce the problem, including improving training for new recruits and temporary staff and introducing better equipment in local delivery offices."
Royal Mail has faced attacks over pricing policy, single deliveries and the conduct of some staff in the past fortnight.