The postal service will take weeks to return to normal following the strike, says the Royal Mail.
Returning staff face a mass of mail
Tens of millions of letters have accumulated in the backlog caused by weeks of disruption in some areas.
Many staff are returning to work on Monday after leaving picket lines, to set about clearing the backlog.
But a Royal Mail spokesman said letter boxes would remain sealed for now and he hoped normal service would resume before the end of the month.
A Royal Mail spokesman told BBC News Online: "We probably have letters in the high tens of millions to get through.
"We will have to start working through them but we'll keep letter boxes sealed for the time being.
"Realistically, it will be weeks before a proper service will resume.
"The one-day action in London took three to four days to recover parity."
He said there were staff returning in large numbers already, many without any formal meetings with union officials.
In London, workers at delivery offices in Mount Pleasant and Nine Elms were back at work.
And offices in Dartford, Colchester and Chelmsford were also being staffed again.
So the backlog was being worked at immediately, the spokesman said, although there would be no special arrangements for extra overtime.
More meetings are scheduled for lunchtime, such as in Greenford, west London, where the dispute began.
The spokesman said: "Some offices are in a better position than others, so there will be some movement of mail around different centres."
He said he hoped the service would be back to normal before the busy Christmas period, which begins at the end of November.
The dispute was estimated to be costing the Royal Mail around £10m a day in London alone.