High-volume users who purchase additional IP addresses may assign SMTP usernames, authenticated email / IP addresses, and API keys to an IP pool. This will help spread email load among the IPs in the IP pool.
To do this, create the necessary pools via the "Settings > Dedicated IPs" section of the user dashboard by selecting the "Add New Pool" button towards the bottom of the page.
Each pool can then be assigned to specific SMTP Users, IP Addresses, or API keys via the following sections of the user dashboard:
Sending > SMTP Users
Sending > IP Authentication
Sending > API Keys
Users sending various types of email in bulk should consider creating IP pools as a means to further maximize deliverability. For instance, creating an IP pool for marketing emails and another IP pool for transactional emails is a great way to ensure important email lands in the inbox (transactional mail), while taking measures to improve engagement for other email more prone to getting snagged by spam filters (marketing mail).