High volume email sending is also known as mass email sending.

It refers to marketers that require large send capabilities, which can be in excess of 200,000 subscribers.

It's good to note that sending the same message to everyone is discouraged, since segmentation will offer far more effective results. Also, email senders need to avoid being associated with spam, which can also happen if sending mass campaigns often.

By following best practices, high volume senders can reach the maximum number of users in a way which will deliver campaign success.