In-mail is a private message feature on the LinkedIn platform. It is a paid messaging feature unless the recipient is a premium member who belongs to the OpenLink Network. In this feature, a person can send a private message to a LinkedIn member who is not in his/her connection. It is an email ad on LinkedIn just like an email.
The Inmail message gets immediately delivered to the recipient’s web-based Inbox on the LinkedIn account. The sender never learns the recipient’s address, so both parties have some privacy. The recipient can then view your profile and decide whether to respond. The cost of Inmail differs for normal and premium members and different target audiences.