Halakhah zu Könige II 4:26

One may not send regards to a woman whatsoever, even by means of a messenger, and even by means of her husband it is forbidden to send her regards, but it is permitted to ask her husband of her welfare. And the reason that one may not send regards to a woman is because maybe through sending her regards, even through a messenger, they will become acquainted with one another and come to affection [Rashi]. And according to this, the prohibition is only with sending regards that causes mental closeness and love, but to say "good morning" or the like - it seems that it isn't forbidden. And to ask someone else how a certain woman is doing, some say that this is only allowed by means of her husband and not by means of anyone else [Chelkat Mechokek], and some permit it [Bach]. And to ask a woman about the welfare of another woman is allowed [Maharsha]; and there are those who want to forbid this because the voice of a woman is nakedness, but that isn't seemly, because normal speech isn't prohibited with women, only a sweet voice where the listener benefits from it. And to write in a letter "send regards to your spouse" or "and tell your wife greetings" - it is prohibited, because this is sending regards through her husband, but to ask about her welfare in a letter - it is permitted, because he is asking her husband how she is faring. And as to his daughter and his daughter-in-law and his sister - it seems that it isn't forbidden whatsoever to send regards to them because causing mental closeness isn't pertinent, because it is the way of the land that a father sends regards to his children, and it is an obligation on him, and likewise a brother to his sister. And the bottom line of these concepts is that everything is dependent on astuteness and fear of heaven, and if his impulses are submissive and compliant to him and nothing gets aroused in his heart whatsoever - there isn't a concern about sending regards [Ritva], and therefore we found by Elisha that he said to his attendant "run now to call her and tell her 'peace to you'", although from there there is no proof, because everything was done because of (prophetic) speech, for he was a prophet.