Shaykh Ibn Jibreen (may Allaah preserve him) was asked: What is the ruling on correspondence between young men and women, if this correspondence is free from immorality and love?

He replied: 

It is not permissible for any man to correspond with a woman who is not his mahram, because of the fitnah (temptation) involved in that. A person may think that there is no fitnah, but the Shaytaan will keep trying until he tempts him. The Prophet (peace and blessings of Allaah be upon him) commanded those who heard of the Dajjaal to keep away from him, and said that a man may approach him as a believer, but the Dajjaal will keep trying until he leads him astray.

Correspondence between young men and women involves a great deal of fitnah and danger, so we must avoid it, even though the questioner says that this correspondence is free from immorality and love.

From Fataawa al-Mar’ah, compiled by Muhammad al-Musnad, p. 96. 

Shaykh ‘Abd-Allaah al-Jibreen also said, when he was asked about corresponding with a non-mahram woman: 

This action is not permissible, because it will provoke desire between them and will make them want to meet one another. This kind of correspondence often causes fitnah (temptation) and plants the seeds of zina in the heart, which leads to evil actions. So we advise those who are seeking that which is in their own best interests and who wish to protect themselves to avoid writing to or speaking with non-mahrams, etc., so as to protect their religious commitment and their honour, and Allaah is the Source of help.

Fataawa al-Mar’ah al-Muslimah, 2/578, 579