The IMDb message board system allows registered users to send private messages to any other user of these boards. Rather than being messages that are posted in the board threads that are visible to all users, these are sent to the user's private message mailbox.
The private messages are internal to IMDb only. You cannot send e-mail from the private message section out to internet e-mail addresses. Only registered users who have visited the message boards or posted comments have private message mailboxes, and if they don't have notifications set or don't check their mail often, they may not know they have a private message from you.
Important: Private Messages in your inbox are automatically expired (i.e. deleted) after 21 days.