Large volumes of email can cause technical issues when they're all sent at once. Some Internet Services Providers (ISP) may also identify the campaign as spam if they receive a very large number of emails from the same source within a short period. To avoid this issue, Mailroom steadily sends out your campaign at a rate of around 200 emails per hour.
This time-delay is important to factor into a campaign, particularly when announcing any time-sensitive offers. For example, if a campaign is started at 9am to announce a limited special on your website at 10am, only the first 200 recipients will receive the email in time – so take this into account when you hit send.