Our servers are set to only allow 800 emails per hour per account to be sent. All email above this amount will be discarded by the server. So if you have a list that is 1000 emails and you use the newsletter manager in Zen Cart to send those emails only 800 of them will be delivered and the rest will be discarded.

The way around this is to use a outside newsletter manager, such as MailChimp. Other newletter mangement software applications are PhpList and Pommo