Implied Question: When the Gemara in ha'Socher es ha'Po'alim (Bava Metzi'a, Daf 87a), learns from the Pasuk (in Vayeira) "Ayeh Sarah Ishte'cha" - that one asks after a woman via her husband ...

Answer: That is specifically with regards to asking after her wellbeing, but to send her greetings is prohibited even via her husband,

Reason #1: Rashi (See Hagahos ve'Tziyunim) - Because they are not conversant with the Mitzvos and therefore bring punishment on the people.

Reason #2: Moreover, Yisrael learn from their deeds.

Reason #3: Others attribute it to the principle "All Yisrael are responsible for one another' (Sanhedrin, 27b).

Refutation: This is not correct however, since they did not accept responsibility for the Geirim who accepted the Torah ...

Source: ... as the Gemara concludes in Sotah (Daf 37b) 'It transpires that each person in Yisrael has six hundred and three thousand, five hundred and fifty covenants - seeing as they are all responsible for one another ...

Source (cont.): ... from which we see that that they did not accept responsibility for Geirim, since a large amount of Eirev Rav went out with them. Until here are the words of Rashi - (See Hagahos ve'Tziyunim, 20).

Reason #4: Others explain 'Kashin Geirim le'Yisrael ke'Sapachas' - because Hakadosh-Baruch-Hu warns against hurting them in twenty-four places, and it is impossible to avoid doing so ...