As standard, we have a limitation in place on shared hosting accounts that they can send a maximum of 1000 messages per hour. This limit is in place as few websites sent more than this volume of mail and so this limits issues caused by hosting account exploits should you not have kept a script or your website fully up to date.
Should you be running a larger mailing list than this and so need to send more than this amount, we can certainly allow this. Please just open a support ticket as we can override this on a per account basis subject to a few checks being made with you.