The MP for Stratford is to meet Royal Mail officials after delays to deliveries in parts of Warwickshire.
The firm said new national working arrangements were affecting the Stratford depot which caused disruption in Warwick, Rugby and Kenilworth.
But Conservative MP, Nadhim Zahawi, said the delays were unacceptable. He will meet the regional director later.
A spokesman apologised to customers and said changes to the way deliveries were made had caused some disruption.
Employees had been working hard to restore services to normal particularly as the volume of mail increases at this time of year, Royal Mail added.
Need to 'adjust'
Mr Zahawi told BBC News that he had received numerous letters of complaint.
"I have had more e-mails and letters about this than probably any other subject that we have been considering in this Parliament," he said.
"It's very concerning. Something has gone wrong and we need to fix it."
In a statement, Royal Mail said: "We would like to reassure customers that following major changes to our operations at the Stratford Upon Avon delivery office, and disruption to our service, mail is being delivered to all addresses across the area on a daily basis.
"We apologise to customers for the inconvenience caused, but when any such major change is implemented some disruption is possible as we adjust to new ways of working.
"All of our employees at Stratford have been working hard to restore services to normal, and will continue to do so to ensure the increased volumes of mail at this time of year reach customers as quickly as possible."