This option controls what happens when a mailbox that is close to or over its disk quota receives email that would put it over quota. By default Virtualmin will bounce the email back to the sender - but if you select the Queue option, email will be held in the local mail queue and re-tried at regular intervals.

If the user's disk usage eventually drops below quota, the message will be delivered. However, if usage never drops the message will be bounced back to the sender after a period configured by the mail server (typically 5 days).