Voting by correspondence (mail) is a version of the advanced vote, and its purpose is to facilitate or remove, as far as possible, the obstacles that prevent the exercise of the fundamental right of suffrage (Santolaya, 1991: 85). That means that it is provided to voters the possibility to issue the ballot via mail, and this is computed along with the results of the votes cast on-site during the election day.

Vote-by-mail systems could have different peculiarities; however, they share common characteristics. First, there should be a formal request for voting by mail by the elector. Normally, the law fixes a date to allow the voters to receive ballots and send them back. In addition, sometimes it could be necessary to add a statement in which the elector certifies his identity and his legal capacity to vote.

On the other hand, after the application for voting by mail, the State must provide the applicants the necessary material so as to ensure their freedom of choice among all the submitted candidacies, and that the sending and receiving of his vote shall be carried out with all the guarantees. It is also recommended to reconcile early voting materials requested, issued, unused and returned (Red de Conocimientos Electorales).

It is the most widespread modality of advanced voting, and this vote may have been issued from a different country to the one in which the elections are held. It would then be a vote from abroad. This modality of voting entails several difficulties (Nohlen, 2007: 251), among which might be noted:

-The definition of the population registered abroad. Realize its magnitude, its location, its characteristics, as well as to ensure access to the vote may involve difficulties.

-The conditions about the concept of citizenship, and therefore, determining who has the right to vote and stand. In this sense, determining if registered foreign citizens have the right to vote, and even if they can stand as candidates, can provoke disputes that could even be solved by the courts.

-Assessment and preparation of resources abroad to correct and effective casting the vote. In this sense, it is necessary to provide the number of ballots needed and that these reach different destinations, opening and adjustment of spaces – usually consular buildings – which will promote the information or the on-site voting procedure, etc.

-The ability to ensure equity in the access and exercise of the right of suffrage of residents abroad.

-In any case, it is to ensure that the vote is issued with appropriate safeguards of freedom, secret and effectiveness, as well as it is issued, it arrives and it can be posted in time (Santolaya, 1991: 86).

On the other hand, voting by mail raises certain problems related to the fact that voters are not present at this time before the representatives of electoral administration and parties. In this sense, it is not guaranteed that who emits the vote effectively is the voter, nor that he casses it in the absence of any kind of coercion or intimidation. The rules of verification of identity and supervision of the free vote cannot be met in these cases.

At the same time, to cast the vote by mail, representatives of political parties are not present, or it is depleted in general election campaign. Also, sometimes nor is able to offer complete security on the transport of ballots, or the voter has the opportunity to effectively check arrival and computation in the count.

Some electoral systems allow the option to vote by mail without any requirement or justification required. Nevertheless, and due to the fact that this type of suffrage involves an additional economic cost, in other schemes is limited to those voters who have certain limitations to voting on-site during the election day. Some requirements which may be set in this regard are: the voter has some job duties that keep him from going to vote that day, or that his religious beliefs prevent him from voting, he is incapacitated or in charge of dependents, etc.

This type of voting is certainly inappropriate for those systems that allow parties to register last-minute changes in the candidates who stand for the elections, as it would diminishing or even cancelling the vote already cast in such circumstances.

In the case of voting by mail, the period for receipt of ballots casted can vary between the same day of the election day, when polling stations close, or after this closure. This allows the effective arrival of ballots that were emitted and which could have suffered some type of retention, to avoid any limitation of the right to vote.