Post workers empty 12,000 postboxes in rural areas across the UK, and the company says the new collection times will affect most of these.
Rob Gibson, SNP MSP for Caithness, Sutherland and Ross, said the changes would make life more difficult for rural residents, and could hit businesses.
He said: "Obviously moving collection times makes it more and more difficult for people to do business. This takes a couple of hours out of the working week for people who rely on the post. I do not see how this is going to improve the service, and it will have knock-on effects for people receiving mail."
Royal Mail said about 45,000 postboxes would move to earlier collections but there would still be a late posting box within half a mile of areas affected and there would be 2000 new postboxes.
It said many postboxes no longer paid their way, adding: "Royal Mail will ensure their viability by improving the efficiency of its collections arrangements."