Npower is TNT's first utility sector client and follows Powergen's move to private mail operator UK Mail.
Npower has more than 6m residential and business customers and the deal with TNT Mail is expected to cover around 50m items annually.
Hilary O'Grady, the energy supplier's logistics manager, said: "TNT has the size, scale and resources to deliver on what it promises."
It will use TNT's Premier service, providing collection and delivery, using Royal Mail's network for final sort and "last mile" delivery, in 48 hours.
However, TNT is looking at hiring large numbers of postmen to provide an end-to-end service in the future, it has emerged.
Peter Bakker, the chief executive of TNT's parent company TPG, said in an interview with the Daily Telegraph that this was one of the business models TNT is looking at, and it expects to have a strategy in place within the next 12 months.
Although it would not have the scale of Royal Mail's delivery network, it would partially free TNT from paying Royal Mail for access.
Bakker added he believed there will be "one or two companies with competing networks to Royal Mail" after the postal market is opened fully to competition in January.
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