A new Royal Mail parcel "super hub" looks to speed up deliveries by processing items in just seven minutes.
The 53-acre site in Daventry, Northamptonshire, is the company's largest automated parcel facility.
In May, regulator Ofcom said it would investigate Royal Mail for failing to meet its delivery targets.
Grant McPherson, from the firm, said it had been "a difficult year... but we're pretty confident now that those times are behind us".
The facility, which opened on Friday at Daventry International Rail Freight Terminal (DIRFT), has the capability to process up to 90,000 items an hour.
Mr McPherson, Royal Mail's chief operating officer, said it meant those sending parcels "can give us huge volumes", which can then be delivered "even quicker than we do today".
However, Citizen's Advice said 15.7 million people experienced post delays in the past month alone.
It said more than half had "experienced negative consequences as a result, such as missing health appointments, fines or bills".
At the moment, only one train a day will operate at the site, but Royal Mail hopes to increase this
Mr McPherson said: "With this facility we will be improving our service to everybody, but we take [delays] very seriously.
"I can guarantee we are focused on making sure our customers get the service that they pay for."
He said letters were "still important to us and they will be in the future", but the number sent was declining while the amount of parcels was increasing.
"People are ordering more and more things online and we should be ready to serve those customers as well," he added.
