When you send newsletters to your clients and subscribers, they not always land in their mailboxes, for various reasons. For example, a mailbox may be full, temporarily suspended, the mail server may be done during maintenance, etc. In this case an email message is not delivered but is bounced, i.e. returned to you, accompanied by an appropriate error message.
Mailer app in your Webasyst account can track and display information about bounced emails. To enable tracking of error messages returned together with bounced emails, set up a Return-path address. It must be a separate (auxiliary) mailbox which you do not need for receiving ordinary mail. This mailbox will be used for automatic reading of non-delivery notifications from recipients' mail servers. Mailer app automatically processes such notifications and displays their contents in a convenient form: