Most major internet providers (ISP) and hosting providers (ESP) (ODI is a hosting provider) have restrictions regarding mass emails. Much of our current standards regarding email--specifically mass emails--were first elucidated in the mid to late 1990's by states. The federal government tried to address email and spam with the CAN-SPAM act of 2003.
Certain actions will trigger responses from our email servers which will prevent you from sending and in some cases receiving emails if you unknowingly or knowingly violate polices regarding email usage. For example, at ODI our limits are no more than 300 emails per hour. The server is also setup to take action when there is a high failure rate with your emails. Typically if your failure rate is more than 70%; the server will block you.
A failure would be an address that doesn't go through or resolve correctly--the email is not delivered to the recipient for various reasons. Most email servers, when they see a high failure rate, make the assumption that this is spam. This often results in the server blocking access to a user's email account, locking a user out, or blacklisting that user's email. On the national and international level, an email server's IP address may be blacklisted thus preventing ALL users on that server access because of the actions of one client.
Email policies seek to prevent these extremes from occurring.
Additionally, it is best to use a mass email service, such as MailChimp, if you plan to send emails to large groups (more than 50 emails at a time). You also have the option of using a feature built into our ODI mail servers called "Mailman."