Pixelmon has had the addition of letters and boxes for a while now but has never really utilized them. This usually ends up being an annoyance to players and servers as they are usually discarded all over as junk items. My suggestion provides value to these items and encourages offline communication among players outside of the /mail command found in many essential plugins.
The idea for letters is simple. Simply create a letter using a "PokeMail" and seal it.
Drop the sealed letter in a mail drop box. When you click on the drop box, it opens an inventory and allows you to specify the player to receive the item. The text would autocomplete with players who have an assigned mailbox. If no player with an assigned mailbox exists, it would reject the mail item.
When a player crafts and then places a mailbox, the mailbox auto assigns to the player much like ranch blocks, only allowing that player to check the mailbox. It may be wise to offer a permission for staff to also access these mail boxes.
The same would apply to boxes as well but whats new here is the boxes would have the ability to be closed (already possible) and then sealed. Once sealed, the only way to get the item inside would be to break the box.
Mail dropboxes will only allow sealed packages to be sent. The packages would send to players mailboxes whether or not they are online.
Mail dropboxes and mailboxes would be crafted such as follows: