The Royal Mail says it will take at least a fortnight to clear the backlog of items left over from the unofficial postal strikes.
Sealed post boxes have started to be unblocked
Thousands of postal workers went back to work on Monday, after talks produced a deal between the Royal Mail and union leaders.
All staff are expected to have returned to work by Tuesday morning.
Officials from both sides will go the conciliation service, Acas, to resolve outstanding issues in the coming days.
Millions of letters have been held up by the two weeks of action, and there is now a vast backlog of undelivered mail.
Special Delivery services are still suspended in some areas, although these are due to resume on Tuesday.
Post boxes in London which had been sealed were being unblocked from Monday afternoon, although Royal Mail said that process could take a couple of days.
Downing Street issued a statement welcoming the deal, but warning that more needed to be done.
HOW BAD IS THE DISRUPTION NOW?
Staff have begun returning to work - a full complement expected by Tuesday
Post, especially in London, likely to remain disrupted for two to three weeks because of a backlog of millions of items
Sealed post boxes in London being unsealed from Monday
Special Delivery services, suspended in some areas, to resume on Tuesday. Items in backlog treated as priority
"Yes, this does mark progress but there are still things that need to
be worked through and we obviously hope they will be," it said.
"It's important for customers, employers and employees that this is resolved
as soon as possible."
Trade and Industry Secretary Patricia Hewitt defended the government for not getting more involved.
"It is not for the government to try to
micro-manage the Royal Mail or get involved in the negotiations," she told BBC Radio 4's PM show.
"I have made it very clear that I expect the management and the
union to work together and to create an effective partnership in the Royal Mail
in place of the frankly appalling relations that existed up until a couple of
"I am very pleased that [the two sides have] reached an agreement on this to stop
these unofficial strikes and get the company back to work."
More than 20,000 workers in London and a dozen regional centres were on strike at the peak of the action.
As the strike was unofficial, there had been fears that some workers may refuse to return against the advice of their union.
But BBC correspondent Russel Hayes, at Greenford in west London where the strikes began, said the deal "seemed to have gone
down extremely well".
A meeting of workers there voted unanimously to accept the agreement, and activists began clearing away their picket lines, he said.
"There was a great deal of enthusiasm for getting back to work...
"[It was perceived] that the Royal Mail had taken a big step back in allowing them to return on their previous working conditions, and there was a big cheer for that."
CWU deputy general secretary Dave Ward said the causes of the dispute involved many offices and many local working practices.
The Royal Mail's chief executive Adam Crozier has said unofficial strikes cost Royal Mail 66,000 working days in 2001, and it was still considering legal action over the current walkouts.