Designed by BDP, the building scooped the BCO's Best of the Best prize at the council's annual awards ceremony last night (4 October).
The 9,300m 2mixed-use project, which also landed the national and regional awards for its fit-out, has become the new home for BBC workers in the city following the closure of the famous Pebble Mill studios last year.
The BCO judges were impressed by the 'dramatic' four-storey entrance space at the heart of the scheme, as well as the building's floating mezzanine 'gondolas' within the hangar-like shell.
'The Mailbox base space was far from normal and the BBC made a courageous stand to go to this mixed-use wonder,' said the judges.
'Designer BDP's success can be measured against the challenge they faced from Mailbox's mail-sorting ancestry.'
They added: 'The standard of fit-out is very high; all elements display imagination, but also great discipline.'
Speaking about the practice's success, BDP architect director Tim Williams said: 'The enormous constraints of an existing building made the challenge more exciting and the design solution more innovative.
'I am delighted for everyone who worked on the project that the achievement has been so positively acknowledged with the BCO Best of the Best award.'