While the vast majority of postal votes in the had been delivered on time, there were a small number delivered late to the Bangor sorting office.
" Most of the customers were quite willing for us to wait for them to fill in the voting slips "
Royal Mail spokesman
Contingency measures were put into place immediately to ensure those votes reached the polling stations before 2200BST.
Postal staff and managers spent much of the day hand-delivering 300 postal votes to customers on Anglesey.
Staff then waited for customers to fill out their voting slips before delivering the sealed envelopes to the relevant polling stations.
A contact telephone number was left at addresses where customers were not at home.
A Royal Mail spokesman said: "Most of the customers were quite willing for us to wait for them to fill in the voting slips.
"We have also had several phone calls from customers responding to our letter offering to collect the completed forms from them at their homes."
Royal Mail said it apologised to those customers affected and an internal investigation will be carried out.
And, it pointed out that across the UK as a whole, more than £2m postal votes had been successfully delivered.
New rules have made voting by post much easier this time round.
For the first time in a general election, anyone qualified to vote has been entitled to request a postal vote.
It was hoped that the reforms would encourage more people to use their vote, amid mounting evidence of apathy and disillusion with the political process.
Voters have until the conventional polling stations close on election day to return their postal votes.
Before the Representation of the People Act 2000 relaxed the rules, people had to fall into certain, limited categories to qualify for a postal vote.
These included members of the armed services overseas, those working away from home, and the long-term sick.
And decades ago the checks were even more stringent, with applications requiring the supporting signature of a GP, registered nurse, or a magistrate.
Concerns have been expressed that the changes could encourage fraud, enabling people to claim the votes of others by post or proxy, when an elector nominates someone else to vote on his or her behalf.
But the Home Office rejected this, pointing out that serious penalties already exist to punish election fraud.
And fears that recent strikes by postal workers would throw the system into chaos seem to have been unfounded.
In the past four general elections, the number of postal votes has varied between 620,000 and nearly 800,000, representing only about 2% of the votes cast.