Messages in Junk mail folder get removed automatically after 60 days
When a new email address is created from Site Tools a few system folders are created inside it. One of them is named Junk, whose purpose is to be used for unwanted or spam messages.
To prevent your email account from consuming excessive amounts of storage quota, our system automatically deletes all messages in the Junk system folder after they have been there more than 60 days. This only applies to the Junk folder and messages in the other folders of your email account are not affected.
This means that the Junk folder is not suitable for storing messages. You can create as many folders to store your messages as you need to by accessing your email account via Webmail -> Folder actions (three dots above the list of folders) -> Manage folders -> Create.