Mailboxes can be found on many maps, usually in outposts located on the map. In the three major cities, you can find two mailboxes: one is located near the Auction House NPCs and the other is located near the Bank NPCs.
You've Got Mail!Edit
You can see if you currently have any unchecked mail in your mailbox by looking at the the top right corner of the game screen where the Mini-Map is located. On the lower left corner of the map there is a tiny envelope icon which will flash if you have unread mail.
You can use the mail to send letters, items, and money to anyone (except yourself) on your server. When mail is sent, the recipient will be able to receive the mail out of their mailbox almost instantly. There is a base fee of 5 when sending mail, plus an additional fee when sending items which is dependent on the worth of the items being sent.
When you buy or sell items through the Auction House, the items and money will be mailed to you. Also, items that you tried to sell but the time has expired will be sent back to you through the mail.
You can leave mail in your mailbox as a method of storage. There are only 5 pages to the mailbox, but the mailbox can hold more than that amount. To view more of the messages you have to delete the visible ones to allow room for the other messages on the 5 pages. You can keep the mail in your mailbox without the icon flashing, just click on each piece of mail once. That will mark all your mail as read and keep the icon from flashing.
Mail, however, will only stay in your mailbox for a certain amount of days. After this time has passed, the mail will be sent back to its owner. If the time has passed on already returned mail, the mail and any attached money or items will be lost and destroyed.