The Ruling on Correspondence Between the Sexes
Question :
If a man corresponds with an unrelated woman, and they come to love each other, is this action considered to be forbidden?
Answer:
This action is not permissible, because it excites desire between the two, and incites the urge to arrange meetings and telephone calls. Very often, this flirting and correspondence causes trials and temptations, and implants a love of fornication in the heart, which plunges one into committing acts of unlawful sexual intercourse, or causes them. So we advise those who desire to benefit themselves, to protect themselves against correspondence and telephone calls and the like, in order to preserve one's religion and honor. And Allah is the Granter of success.
Source:
Ibn Jibreen
Fatawa Islamiyah, Vol. 5 Pages 120-121