Through the deal, Express Dairies will have responsibility for large bulk letters and heavy package delivery while TNT Mail will be responsible for collection, sales and marketing.
In a statement, TNT has said the deal is part of its ambition to become the main challenger to Royal Mail amid deregulation of the sector.
Tim Smith executive director of Arla Foods UK, which Express is the trading division of, said: "By working together with TNT Mail, Express Dairies will continue to develop and extend our parcel delivery business."
Nick Wells, TNT Mail CEO, added: "The deal with Express Dairies means that TNT Mail can extend its reach and services within the country."
The seven-year licence handed out to Express by Postcomm last year covers bulk mail, as well as consolidation and enhanced document exchange. This replaced an interim contract.
Other firms to gain a similar licence for business delivery include Hays Commercial Services and TPG Post UK.
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