Is it permissible to communicate with a married woman, knowing that this communication began before the marriage of both parties and continued thereafter? And what is the ruling on being so preoccupied with her to the extent of sending a telegram after exchanging glances?

Addressing and looking lustfully at a non-mahram woman is forbidden, based on Allah's saying: "Tell the believing men to reduce [some] of their vision and guard their private parts." This act is a means to a greater forbidden act. The prohibition increases if the woman is married, as it involves violating the sanctity of her husband and defiling his honor. If the man himself is married, he is held more accountable, and it involves corrupting the woman against her husband. Repentance from this act is obligatory, and one must divert their thoughts from this woman.