There isn't any "inbox", "messages" or anything similar here. When you send a message to a contributor it simply sends them an email containing your message and your email address for them to respond to. It is up to them if they want to respond to your message.
Many people send messages without any useful information (especially links to their questions) and use different names to their MATLAB user names, making it impossible to figure out who they are and what they are referring to. I usually delete those emails.