Sub-accounts allow you to separate your mailings across different API Keys. By default, all accounts come with one active (Master) API Key where all mailings are sent through. You also have the possibility to create a second (Sub-Account) API Key for your transactional mailings for example.
Recommended best practices:
If you are sending both marketing and transactional emails on your account, you should use one API Key to send your transactional emails and another API Key for your newsletters. In the event that one API Key has an issue (for example, a sending rate limit on your marketing campaigns), it will not interrupt the sending of your transactional emails, and vice versa.
If you manage a number of customer’s mailings, you can assign a different API Key to each customer (or even 2 API Keys to each customer if they send both marketing & transactional emails). Should an issue arise with one customer’s mailings (a rate limit or sending is temporarily blocked due to abuse complaints), it will only impact their one API Key, and not all of your customers.
You can also separate a mailing type, recipient list, or a particular customer’s mailing to a separate API Key and give another person access to that specific sub-account with Account Sharing.