An Post today welcomed regulator figures today showing a 4 per cent improvement in its next-day delivery.
Comreg's figures for the first quarter of the year showed 78 per cent of single-piece mail was delivered the day after posting.
Comreg
But the regulator noted the figure was still "well short of the quality service target" of 94 per cent.
"Of note once again is the poor performance of mail posted outside of Dublin for delivery to addresses in Dublin county - 72 per cent, in contrast with the level of service attributed to mail posted outside of Dublin for delivery within county of posting - 81 per cent."
The figures also showed that mail posted for delivery anywhere in the State within three days was 1.5 per cent below target at 98 per cent.
The ComReg report, based on research by Tnsi/mrbi, said: "While performance of all mail flows falls short of the target set by ComReg there is nevertheless a marked improvement in the performance of Dublin mail for local delivery in this reporting period.
"The improved performance will be welcomed by all postal users and suggests that benefits are accruing from targeted and consistent actions by An Post Management in the area of quality in recent times."
An Post chief executive Donal O'Connell said Comreg's findings were supported by recent figures from the International Postal Corporation which showed a 5 per cent improvement in next-day delivery for incoming international mail.
An Post would achieve 80 per cent next-day delivery this year and would hit Comreg's target of 94 per cent within three years, Mr Connell added.
Labour Party communications spokesman Tommy Broughan said An Post appeared to still be a "long way away" from achieving the ComReg targets.
"These figures confirm complaints that I receive on an ongoing basis from families and businesses about slow mail delivery times and unacceptable delays," Mr Broughan said.
He blamed what he said was the company's failure to provide proper holiday and illness cover for staff.