Hard to say since you don't include your version of Outlook or type of mail
account.
Since Outlook itself doesn't have a quota, other than the PST size if you
use an older version, you should contact your ISP to see what limit you're
allowed or if you have an Exchange account, contact that Admin to see what
limit is imposed on your mailbox.
I would suspect if you're receiving a notification (perhaps it's via email?)
that you're nearing your quota or have exceeded it, you should contact your
ISP or Exchange Admin regarding this (depending upon the type of mail account
you have).
