Before you can send out any emails through Soapbox Mailer, Amazon SES requires that you verify your sender emails (they want to make sure you are who you say you are after all!). There are two ways to do this: 1) individually per sender email or 2) by domain. If you have a lot of sender emails or if they change fairly frequently, verifying your domain may be preferable so sender emails don't need to be individually configured.
In order to verify your domain, you must have access to the sender domains DNS settings so you can add a TXT record.
Note: After your domain has been verified, you will still need to setup the notification topics for your senders to make sure bounce and complaint responses get posted back to Salesforce. You will only need to do this once to apply to all emails that use your domain.
Note: While verifying your domain will apply to all users, individual senders will still need to be created in Salesforce in order for Mailer to use. For guidance on how to do this, please see Step 2 here.