When using Soapbox Mailer, all of your email is being delivered by the good folks at Amazon via their Amazon Simple Email Service (SES). They have a few different quotas (limits) on their services, based on your account status with them. One of the key quotas is your email sending limit.
Amazon SES sets your sending limit based on a daily (24 hour) rolling window. So, they give you a certain number of emails you can send in a rolling 24 hour period. Want to find your daily sending quota?
1) Go to the Amazon Web Services console: https://console.aws.amazon.com/console/home
3) Click on the link to SES: https://console.aws.amazon.com/ses/home
4) Look at "Your Amazon SES Sending Limits", specifically "Sending Quota"