Some 2.9 million information packs for registered voters are in the post. The 2008 general election is on 8 November.
The packs include an Easy Vote card, which people can take with them to polling booths.
The Chief Electoral Officer, Robert Peden, says each card has voter information that will help personnel at polling stations issue ballot papers.
He says the mail-out is even bigger than the one for the last election, which involved 2.8 million packs.
Mr Peden says about 188,000 eligible voters are still not enrolled, including about 85,000 people aged 18 to 24 years.
People can enrol until 7 November, the day before the election.