When an email promo cannot be delivered to an email address, it's called a bounce. There are lots of different reasons why promos can bounce so, when it happens, a "return to sender" message (or, SMTP reply) is sent from the recipient's mail server to explain why. Promo.ly will display this information in your activity log. We recommend you maintain your mailing list every couple months to ensure deliverability remains high. If you would like your email list cleaned (to check for dud email addresses) please raise a support ticket for a quote. We can clean up to 25,000 email addresses at once.