Mailbox
A Mailbox or the Mailbox-system is a feature you can use to sent messages and various items to other players. Mailboxe's are located near most Poké-Center in the game. The feature was added in 2017.
Requirements and Box-locations

In order to be able to use the Mailboxe's, you need at least 4 Kanto badges. The fourth badge is obtainable in Celadon City. Mailboxes are usually located outside, right next to Poké-Center with only a very few exceptions.
How to sent a mail
In order to sent a mail you first have to interact with any Mailbox, then a mailing overview will open. You can here review any old mail you received and sent new mail via the 'Send Mail'-Button. Once this button is clicked, a new window will open. Here you will have to enter a mailing title, enter a recipient aka a valid username and some content into the mail message field. Optionally you can drag-and-drop some items out of your bad into the 'Attach-Item'-bar. Not every item can be put there. A message in the chat window will tell you if the item you tried, and failed, to place can't be mailed.
