Downstream access

Downstream access (DSA) is mail that has been collected and distributed by a competitor, but is handed over to Royal Mail mail centres for final processing onto local delivery offices, where they are delivered.
History.
The ability to utilise the Royal Mail network in this way (analogous to local loop unbundling in the telecoms sector) was first introduced in 2003, ending a 350-year monopoly.
At first, there were strict limits, restricting DSA to large operations, however over time these have been relaxed further.
With the final abolition of the Royal Mail monopoly (but not its universal service obligation), DSA mail makes up an ever-increasing proportion of mail received in the UK. However 99.8 percent of the mail delivered is delivered to Royal Mail to deliver for these third party suppliers. Government attempts to show competition in the postal sector have not created competition as such, but Royal Mail has lost some of the easier and more profitable section of its business. The Postal Services Commission restricts the prices at which Royal Mail sell this downstream access to third party companies.
Future developments.
Increasingly, large mailers (banks etc.) are looking to work with or sidestep the DSA providers, and obtain direct access (Customer Direct Access) to Royal Mail delivery themselves.
With the end of the monopoly, some DSA providers are moving into the delivery market in major conurbations. Such bypass mail never touches the Royal Mail network.
Another innovation is the emergence from DSA providers of so-called Hybrid mail services, which take electronic documents, produce real mailpieces & deliver them to their final destination.
DSA Providers in the UK:
References.
Citipost Mail