Royal Mail has sold six acres of its north London sorting office to housing developers for £193.5m.
The company exchanged contracts with house builders Taylor Wimpey on the sale of its Mount Pleasant offices.
Royal Mail said it would make "significant further investment" to to separate a retained office site from the housing plots.
The sale will "help us secure the long-term future of our key central London operational site," Royal Mail said.
Planning consent was secured in March 2015 to create 681 residential units - as well as retail, office space and public space - on the site.
A further 0.27 acre plot, the office element of the planning consent, will be marketed at a later date.
Separating the two sites is expected to be completed by 2021, at a cost of £100m.
Martin Gafsen, group director of property and facilities solutions at Royal Mail, said: "The sale of our development sites at Mount Pleasant to Taylor Wimpey is a great opportunity for us to contribute to the regeneration of the area around our iconic Mail Centre building.
"This will create more housing, including affordable units, as well as community facilities and more public space.
"It will also help us secure the long-term future of our key central London operational site."