What is the suppression list for?

Think of it as a do-not-email list. Every account has one and subscribers are typically added to the list because they have been unsubscribed, or emails sent to the email address have bounced.

With these inactive subscribers recorded in your account as "suppressed" it helps to prevent any further emails being sent to them. This keeps list sizes in check and helps to protect your sender reputation.
Automatic

Any subscriber who meets the following criteria is added to the suppression list automatically:

• They unsubscribe themselves using a form or unsubscribe link in an email. Or, their status is changed to unsubscribed through your account.
• They mark one of your emails as spam.
• An email sent to the subscriber results in a hard bounce.
• The subscriber's email address returns a series of soft bounces, resulting in a conversion to a hard bounce.

Manual

Adding email addresses manually is good practice to ensure that people marked "do not email" in an external system – for example, a CRM tool – are not imported to a list accidentally.

To add domain or individual email subscribers go to Subscriber Management under the Administrator.