One of the aspects that helps email be successfully delivered is your sending reputation. Your sending reputation is how much trust email service providers such as Google, Yahoo, Microsoft, etc, have in you, and if they'll let your emails through or not.
This is affected by a huge variety of factors, and they all use different metrics to determine how trustworthy you are, however some of the main factors are:
Your bounce rate
Your spam complaint rate
Your open and reply rate
The content of your email and subject (does it sound spammy)
One bad send can have a huge detrimental affect to your sending reputation, so to protect you and others on our network, we have limits for new customers and several safeguards in place to protect your, and our, reputation from bad sends.
We don't enforce a limit on the amount of emails you can send per month, but we do limit the total amount of recipients in a given bulk email. This limit will grow over time as you use the system and your reputation improves.