Letters in the in-game Mailbox are kept for 5 days or till their number riches 1000. After that the outdated letters or those that exceed the limit are deleted.
If you want to save any messages, move them to the Archive.
Messages are not deleted in the Archive by the system. Only you can delete messages from the Archive.
There is limit of the messages that can be kept in the Archive. It is impossible to move more letters to the Archive than the limit allows.
All answers to the FAQ