Turning this option off makes sense only if you want to use this address as a mail forwarder, which will forward all incoming emails to another address.
If you leave the Mailbox option selected, specify the mailbox size or use the default size defined by your provider policy or service plan.
Here's how to disable Mailbox on the Plesk control panel:
Login To Plesk.
Please click Mail > Then you select the email address (email_address).
Uncheck the Mailbox option and click OK
Notes: Disabling a mailbox does not affect (delete) its content.
