The final mail collection in some parts of north Derbyshire is being made too early, according to residents.
Sometimes letters posted mid-morning miss the collection
Postal regulator Postcomm has reported that some post boxes in Buxton are being emptied as early as 9am.
Royal Mail said it collected the post early in some locations so it could meet its targets for next day delivery from more remote places.
Mail posted mid-morning sometimes stays in the post box until the next day, PostWatch North's Hilary Putnam said.
"It makes it difficult for businesses or anybody who needs to deal with their mail urgently," Mrs Putnam said.
"In some places the last delivery time is getting later - so in some cases the last collection is before the last delivery."
A Royal Mail statement said: "Where we have to do an early collection from a particular box, we always aim to ensure that there's a later collection made at a nearby box or Post Office branch.
"None of our rivals provide collections and deliveries nationwide to the UK's 27m addresses or have the capability to do so."
Helena Stubbs from Derbyshire's Rural Community Council, said: "It is quite incredible. People living in isolated areas should not be disadvantaged because they are living in remote parts of the countryside."
The Postcomm research is part of a consultation to see whether Royal Mail is collecting and delivering letters and parcels at times that suit customers.
Under Royal Mail's "universal service obligation", letters and parcels must be collected at least once a day from every post box and delivered once a day to every home and business.