An email suppression list list — aka a suppression mail list — is a list of email addresses of people who have either opted-out of or have unsubscribed from your organization’s email marketing programs. Basically, you use a suppression list in conjunction with email marketing programs so you DO NOT contact these recipients. Most email marketing software has the functionality to allow you to maintain this list separate from your recipient lists. (A marketing pro-tip: Up-to-date unsubscribe and opt-out information should also be recorded in your organization’s CRM system so other parts of the organization always has access to this information.)
In addition, your organization’s email suppression list should be shared in the following situations: if you use a third-party provider for email marketing or if a partner executes a marketing campaign on your behalf. In these cases, you would provide your email suppression list prior to the launch of any marketing campaign so those suppressed email addresses can be scrubbed.
In the US, the CAN-SPAM Act allows organizations to take up to 10 business days to add unsubscribes and opt-outs to the suppression list, so it’s in your best interest to keep the list updated as soon as possible, if not in real time.