Talking between non-mahram men and women is basically not forbidden if the talk meets the conditions set by the syara '. Such talks are good, keeping the courtesy of courtesy, not causing slander and no khalwat. So if things that are important or degree eg buying and selling things, fires, illness and so it is not haram.
In history we see that the wives of the Messenger of Allah spoke with the Companions, while answering the question they raised about religious law. There is even between the wife of the Prophet SAW who became the teachers of the companions after the death of the King Aisha RA.
In this case, Allah Almighty says that means: "Therefore do not you (the wives of the Messenger) submit (ie soften the voice) in speaking so that the person in whose heart there is illness has a bad desire. But say good words ". (Surah al-Ahzab: 32)
Imam Qurtubi interprets the word 'Takhdha'na' (subordinate) in the above verse with another meaning qaul (softening the voice) which gives a feeling of bonding in the heart. It is forbidden in our religion to attract the hearts of those who hear it or to read it.
This means that prohibited conversation is the talk that causes slander by softening the sound. Included here are words expressed in written form. Because with the writings of a person can also express the words that cause a person to feel a special relationship, then cause a bad desire.
Included also in softening the voice are words or gestures that are good, but he may cause slander. That is the way and the form that causes the emergence of special feelings or desires that are not good on the other person who is not a mahram. Either by voice or through writing.