MailBox is pretty simple to use. The first thing you'll need to do is make a mailbox. Get a fencepost and put a chest on top of it where you want your mailbox to be. Then, type "/mailbox create" and you'll have a prompt telling you to left click your mailbox. Left click your mailbox. Now you have a place that receives mail.
If you want to check your mail, you have to manually open the chest to see. If you're logged in and someone sends you something, you'll get a message telling you that you have a new package in your mailbox. If you aren't logged in you will not receive the message so it is good to check your box if you are expecting something.
The message that tells you whether or not you have mail when you log in is based on Essentials messaging. It is not the same as MailBox.
If you want to send mail, they have to have a mailbox set up (which pretty much everyone does on the server, unless you're new). In order to do that, you have to have the object you want to send in your hand and type "/msend (player name)" where, obviously, (player name) is who you're sending it to.
If you get lost and don't know what to do, thankfully the command "/mailbox help" is absolutely no help whatsoever. Pretty much everything you need to know about it is in this post. There are no fees for sending packages or receiving them because it's broken, but that's okay - free mail is good times for all.