SmartrMail automatically suppresses email addresses that are invalid or have proved undeliverable to keep your list clean and protect your deliverability.
After clicking through a list, you'll see a count of all your subscribers
Suppressed email addresses have proven to be undeliverable due to a hard bounce or multiple soft bounces.
Hard vs. Soft Bounces
Hard bounces are permanent delivery failures that are caused by invalid email addresses. These can often come from brick-and-mortar store contests or mistyped email addresses. Emails that hard bounce are automatically removed from your mailing list with SmartrMail.
Soft bounces are emails that are temporarily undeliverable due to a problem on the email recipient's side. These bounces are caused by full inboxes, offline servers, and emails being too large. SmartrMail will automatically resend soft bounces, but after multiple tries, will record the email as a hard bounce. Once recorded as a hard bounce the email address will be suppressed and removed from your email list.
Suppressed subscribers are not counted on your bill.
Suppressed Subscribers in Shopify Subscriber List
Some Shopify stores see a high number of Not-opted in or Suppressed subscribers in their Shopify Subscriber list. This can be caused by your customers not 'opting-in' for marketing. You can read more about it in Shopify's documentation.
Occasionally stores accidentally upload a CSV subscriber list from customers who have opted-in to receive emails directly into their Shopify store, but mark 'Accepts Marketing = No' by mistake. This will cause these customers to be opted-out of email marketing in Shopify, and they will show in SmartrMail as suppressed. You can fix this by re-uploading the customers into Shopify and making sure the column in the CSV 'Accepts Marketing = Yes' for the customers who have opted-in to receiving emails.
More details on how to do this are here: https://help.shopify.com/manual/customers/import-export-customers