You can send mail to another character of the same school by using the mailbox (shown left). Mailboxes can be found on the school grounds and certain regions next to the stores.
If you receive mail away from a mailbox, click the new mail notification at the bottom of the screen and it will give you an option to open the mail without having to find a mailbox for a small fee.
Open - Opens the selected message, you can also double-click a message to open it.
Return - Returns the mail to the sender, works with certain mails that have items attached.
Delete - Deletes the selected message.
Post - Creates a new message to send to someone.
Below is an example of creating a message and an example of opening a message from someone else.
The sender's window on the left is what you will see when you click the Post icon in your inbox. Enter the character name of the person you want to send it to. Then enter the subject title. Your message can contain up to 100 characters max.
To send an item with the message, click on the item in your inventory. Only certain items can be sent by mail and will cost an extra fee.
Send Money - Enter the amount of Mar you wish to send to the recipient.
When you are done composing the message, click the send icon to send it. Or click the cancel icon to return to your inbox without sending the message (message will not be saved).
The recipient's window on the right is what you see when you open a message. It will show if any item is attached or if money was sent along or requested.
Charged/Wired Amount - Shows how much Mar you need to pay to take an item attached into your inventory (Charged Amount). Will also show how much Mar was sent to you if any (Wired Amount).
There is a time limit to messages kept in your mailbox. Unread messages will be returned to the sender within 2 days (same goes with Auction messages). Read messages will be deleted within 4 days.
You can always check up on the rules and details by choosing the "learn how to use it" option when you double-click the mailbox.