When an email bounces back with a "mailbox full" or over-quota SMTP rejection, the standard assumption is that a subscriber has let thousands of unread messages pile up in their inbox. For Gmail users, that assumption might actually be wrong.As Mark Hachman points out in his PCWorld article, Your Gmail storage problem probably isn’t Gmail, Google uses a single pooled storage quota across Gmail, Google Drive, and Google Photos (and even mobile backups). A subscriber’s inbox could be nowhere near overflowing but a few 4K phone videos or an automated photo backup could cause the user to hit their 15...