This centuries-old tradition survived in the Foreign Office through countless changes of government, upheaval and wars - before coming to an abrupt end under Labour in 2006.
Perhaps the remarkable thing was that it lasted so long.
An outgoing British ambassador had absolute freedom to write whatever they wished in their final telegram home: about the post they were leaving, about the governments they had served, about the Diplomatic Service itself.
Especially candid were final valedictories, written by ambassadors quitting their last posting before retirement.
Diplomats finally had an opportunity to be indiscreet, without fear of reprisal, and many seized it with both hands.
They knew that whatever they chose to say - serious foreign policy advice, funny anecdote, or bitter tirade - it would find the widest audience.
The tradition was that valedictory despatches would be widely circulated, with hundreds of copies printed and avidly read across government. Lord Moran wrote his final telegram from Canada in 1984.
The problem was that with the advent of email, confidentiality became harder to keep.
You can read a selection of despatches here.
The despatch I was most curious about is here. Sadly, if there were any juicy sneers or insults in it they seem to have been excised by Her Majesty's Wielder of the Royal Censor Pen, or whatever the title may be for the Red Taper who redacted Sir Robin Renwick's despatch from Washington in 1995.
One despatch that struck a cord with me was Ambassador Roger Pinsent's from Managua in 1967.
Nicaragua is a land of contrasts. The approaches to the towns are squalid to a degree that shocks the visitor from Europe. On arrival we unwittingly caused some offence by enquiring the name of the first village we passed through on leaving the airport, which turned out to be the capital city of Managua.
What traveler to Central America hasn't been in that situation?