Is a woman's conversation with a man via the internet (messenger) forbidden (haram), whether it be like siblings, or with the intention of marriage with the other party's assurance, or with the intention of treatment with a psychiatrist?

Chatting and getting to know members of the opposite sex online is a pretext and a step from the steps of Satan towards the forbidden. Therefore, it must be blocked and prevented, unless there is a need for it as a means to address those one needs to address, such as a doctor and the like. This also includes the fiancée speaking with her fiancé merely to clarify a matter, in accordance with legitimate Islamic guidelines.