Set the Mailbox Properties File

Edit online

If you write a message to a mailbox and the name matches the name of a message in the mailbox, the service deletes the old message before it adds the new message.

Set the following value in your mailbox.properties file:

disallowDuplicateMessages=true

This ensures that every message in a single mailbox has a unique name. It also ensures that a message and a mailbox do not have the same name.