Praise be to Allaah.
It is known in the religion of Allaah that it is forbidden to follow in the footsteps of the Shaytaan. Everything that could lead a person to fall into haraam things is also haraam, even if in principle it is originally permitted. This is what the scholars call the principle of warding off harm.
Concerning this matter, Allaah says (interpretation of the meaning):
O you who believe! Follow not the footsteps of Shaytaan. [al-Noor 24:21].
With regard to the second matter, He says (interpretation of the meaning):
And insult not those whom they (disbelievers) worship besides Allaah, lest they insult Allaah wrongfully without knowledge [al-Anaam 6:108]
Here Allaah forbids the believers to insult the mushrikeen lest that leads to them insulting the Lord, may He be glorified and exalted.
There are many examples of this principle in shareeah. Ibn al-Qayyim (may Allaah have mercy on him) mentioned many of them and explained them well in his excellent book Alaam al-Muwaqqieen.(See 3/147-171 thereof).
The issue under discussion here also comes under this category. Conversation whether verbally or in writing between men and women is permissible in and of itself, but it may be a way of falling into the traps of the Shaytaan.
Whoever knows that he is somewhat weak, and is afraid that he may fall into the traps of the Shaytaan, has to refrain from such conversations, in order to save himself.
Whoever is sure that he will be able to remain steadfast, then we think that it is permissible in his case, but there are certain conditions:
The conversation should not be allowed to wander too far from the topic being discussed; or it should be for the purposes of calling others to Islam.
They should not let their voices be soft, or use soft and gentle expressions.
They should not ask about personal matters that have no bearing on the matter being discussed, such as how old a person is, how tall he or she is, or where he or she lives etc.
The conversation or correspondence must be halted immediately if the heart starts to stir with feelings of desire.