In many support applications, the user would need to be logged in before they can submit a question. In this sort of scenario, the from column would not be needed, you would use a linking column to link to the users table
SupportID - Primary Key
UserID - relates to users.UserID
ParentID - initially set to 0. when replying, use the SupportID value for the initial support post
In this example, the userID column stores the ID of the logged in user, when the admin responds, it will store the admins UserID. On responding it will store the ID of the first support post in the parent ID column.