1. A woman can speak to men out of necessity, such as for conducting a business transaction or conveying religious knowledge. The wives of the Prophet (may Allah bless him and grant him peace) used to relate knowledge to the Companions.
They used to do it from behind a curtain, and although women other than them are generally not subject to the same strict codes, the scholars have said that if a sister is giving a regular class to brothers (and similarly if a brother is giving a class to sisters), there should be a curtain or some other separation between them.
This is based on experience, for if a class is given on an on-going basis, it is very possible and almost inevitable that some kind of fitnah (e.g. development of some attachment between a brother and a sister) may result in the absence of a curtain. Imam al-Shafi’i had around 200 women teachers, and studied from them from behind a curtain.
2. As for casual conversation, without a need, this is surely opening the door to great fitnah and will almost inevitably lead to adultery of the tongue.
"Allah has written for Adam's son his share of adultery which he commits inevitably. The adultery of the eyes is the sight (to gaze at a forbidden thing), the adultery of the tongue is the talk, and the inner self wishes and desires and the private parts testify all this or deny it." [Reported by Bukhari (8)74:260]
3. If a woman does speak to a man who is neither her husband nor her mahram (permanently non-marriageable kin) then the two may not be alone, for otherwise the third of them is Satan. This applies even if the man is her brother-in-law, and in fact even more so, for the Prophet (may Allah bless him and grant him peace) said, "The brother-in-law is death." [Reported by Bukhari] Further, scholars have deduced that the man and woman may not be alone with a third (non-mahram) man, but they can be with another woman.
4. If a woman does speak to a man (and vice-versa), it should be straightforward speech, for exceeding the limits of necessity opens the door to Satan. Also, the woman should make sure her voice is not soft and alluring.
"And be not soft in speech lest he in whose heart is a disease be moved with desire." [Qur'an, Surah Al-'Ahzab, 33:32]
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