There are two types of messages.
Ones that can be saved permanently and ones that have an expiration period.

Therefore, if a message with an expiration period is not claimed on time, the Items included in the message will expire.
The expiration period is indicated on each individual message.
Messages without an expiration period can be saved permanently.

If a message with an expiration period, that has not yet expired, has disappeared from your Mailbox, please leave us an inquiry to our in-game Customer Service Center (HELP), including the information below.

Content of missing message:
Name and number of Items that were included in the message:
Date and time when the message was received: