Our servers are setup to only allow 200 outbound emails per hour for each hosting account, so If you're trying to send a newsletter please split the delivery into lots of 200.
The reason we don't allow more than 200 outbound email messages per hour, is to reduce the likelihood of our IP addresses being black listed. If one of our server's IP's are black listed, this impacts on the delivery of messages for the entire server your account is on.
Some newsletter software or plugins allow "timed release" which automatically sends emails in lots over a given period of time. Alternatively, you may wish to use an email newsletter provider like Mailchimp or Campaign Monitor.