Does divorce take place between a couple if the husband says to his wife: "You are divorced if you leave," and the message reached her and she read it after she had left, and his intention was to prevent her, not to effect a divorce?

A written divorce does not take effect unless intended by the husband. Since the husband did not intend for the divorce to take effect by writing it, it does not occur. As for the wife, if she returns after her husband's message, there is no sin upon her for obeying his command. If she does not return, she is sinful for persisting in what he forbade her from without his permission.