We have set a limit of 200 outgoing emails per hour on shared and reseller hosting, 500 outgoing emails per hour on
VPS Hosting, 1000 outgoing emails per hour on Semi Dedicated Hosting and 3600 outgoing emails per hour on Dedicated Hosting to avoid any sort of email spamming from our servers. Customers cannot send more than the specified number of emails in an hour irrespective of the size of their mailing lists.
May I know why you wish to send up to 300/Hour, that's very strange...