Mailboxes can be found in most towns and outposts throughout Haradon. These are used to send messages to other players, and to you. who will receive the message the next time they check their own mailbox.
If you have been sent unread mail you will see a 'letter' icon above your combat level.
The recipient does not have to be online to be sent mail, although they must be on the same server as you.
You can also attach money and items (up to 5) to your letter, for the recipient.