A Sub-Mailer is created when you work with a network/agency that also has its own SUPPRESS account. In this scenario, the partner will create a Linked List in their account which allows them to connect to your system at a Suppression List level. Here is an article that speaks a little further about it: What is a Linked List?
You can find a Sub-Mailer in your account by going to a Mailer's profile page and scrolling down to the section called Sub-Mailers. To learn more about the Mailer's Profile page, please review this article: How do I View the Activity of a Affiliate?
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