An Post and the gardaí have launched separate investigations after 20 bags of undelivered post, some dating back to December 2014, were found in the Dundalk.
Gardaí have confirmed they carried out a search of a house earlier this month and uncovered a batch of undelivered post.
The undelivered post included letters for medical appointments, tickets to events, passports and cards containing cash and gifts.
It is believed that between 7,000 and 8,000 individual items of post were discovered.
However, it is also understood that nothing had been taken from the mail and it was, as such, intact.
One person is thought to have been suspended by An Post which is carrying out its own internal enquiry.
The delayed mail is now being delivered with a letter of explanation and apology from An Post accompanying it.
An Post has described the situation as "a serious matter".
A spokesperson said, given the ongoing investigations, it was not in a position to comment any further at this time.