Meanwhile the volume of delayed mail is expected to fall to five million items by the end of the day, Royal Mail said.
Royal Mail had previously said that the backlog was 30 million items.
The CWU had claimed that the backlog was in excess of 100 million items on Saturday morning.
Postal workers staged two days of strikes on Thursday and Friday last week.
The latest talks are taking place at the Trades Union Congress (TUC), which helped to resolve a dispute between the two sides in 2007.
The CWU earlier said it was a "huge, positive change" having Royal Mail managing director Mark Higson at the talks, as he had not been present at negotiations up until now.
Mr Higson had not commented when he arrived at the meeting.
However, the CWU's deputy general secretary Dave Ward had said there were some things the government needed to sort out, including the massive pension deficit at Royal Mail, believed to be about £10bn.
TUC general secretary Brendan Barber says talks between Royal Mail and union leaders were "useful"
"We are very disappointed about some of the comments that Lord Mandelson is making about this dispute. He's not telling the truth about what this dispute is about," Mr Ward told journalists on his way into Congress House - where further talks will also take place.
If this week's strikes go ahead, they are due to involve:
• Thursday - 43,700 staff in mail centres, delivery units in mail centres, network logistic drivers and garage staff walking out from 0400 GMT
• Friday - 400 workers at three sites in Plymouth, Stockport and Stoke, who assist mail centres by reading and entering mail addresses
• Saturday - 77,000 delivery and collection staff across the UK.
Focusing on customers
Martin Shankleman, employment correspondent, BBC News
Both sides are dishing out propaganda. There is a battle on and they are both putting out partial information.
Royal Mail says they were very close to an agreement last week. In fact, at times it says there was an agreement. But the union comes back and says there is no agreement - there was a form of words they never signed up to.
The TUC is often a good way of getting one side off the hook if they want to climb down: it is known as something of a fudge factory. Now judging by what was said on Sunday, it does not seem that either side wants to climb down or is in the mood for compromise.
Amazingly, we had Royal Mail chief executive Adam Crozier telling the union to "shut up" and Billy Hayes of the union telling the company that they were talking nonsense. So it doesn't seem like there is a compromise in the air, but maybe there is something going on behind the scenes.
While both sides agree that job cuts are necessary, they disagree over their extent, and the future pay and working conditions of the workers that remain.
On Sunday, Mr Crozier told the BBC's Andrew Marr show that he hoped an outline deal would see the strikes cancelled.
Mr Crozier denied accusations he was taking a back seat in the dispute and said he was not involved in the direct talks with the union because his focus was instead on talking to affected customers.
He also denied that he was taking any direction from the government.
The CWU said it would not agree to any compulsory redundancies, adding, "we want change with agreement, with job security at the heart of it".
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