Page 128 - 42 Rules for 24-Hour Success on LinkedIn: Practical ideas to help you quickly achieve your desired business success.

InMail
This is another way to contact any user directly on
LinkedIn. However you need to purchase these.
InMails are private messages that enable users to
contact each other with LinkedIn acting as the in-
termediary to ensure that all messages remain
confidential and that contact information is kept
private. If the recipient is not in your network, you
will only see the recipient's name and email
address if the InMail message is accepted.
InMails can be purchased individually or as part of
a premium account. Since senders pay for each
InMail sent, they tend to choose the people they
contact with care. If you use them, understand
that, once they are sent, they expire within 15 days
if not read by the receiver. This is free when con-
tacting an Open Network Member.

InMail Feedback
This is a reputation system LinkedIn uses to
minimize spam. It is based on recipients'
responses to senders' InMail messages. In
general, you are notified only when InMails come
from senders with a 3-star rating or higher. You can
change your preferences in your Contact Settings.
As with Introductions, you can always tell how
many you have left by going to your Account &
Settings page.

112 42 Rules for 24-Hour Success on LinkedIn