Every time you run a bulk email campaign your message may not be delivered to one or more recipient addresses for various technical reasons beyond your control. For example, some of the email addresses in your recipient list may not exist by the time you are sending your message.
In such and other similar cases it does not make any sense to try to send further messages to the same addresses as no one will receive them anyway. Note that your mail server's IP address may be added to “black lists” of various mail services for attempts to run spam-like mailing campaigns. Besides, attempts to send messages to knowingly non-existent (unavailable) email addresses will simply mean a waste of your mail server resources.
Mailer app will save you from both of these issues by automatically filtering out all addresses from the recipient list, which turned out to be unavailable during previous email campaigns. Thus, you may always include any addresses in the recipient list, and Mailer will automatically filter out the unavailable ones in order to send your message to all other recipients.
To make such automatic filtering possible, you must set up a Return-path mailbox (see corresponding field on the “Send” step when creating a new campaign). All non-delivery diagnostic messages from mail servers will be sent to that email address. If you have not set up a Return-path mailbox, then the filtering of unavailable addresses will not work! Be sure to set up a separate mail box for the Return-path address, which you do not use for correspondence. All messages delivered to that email address will be retrieved by Mailer app for further analysis.
The list of filtered email addresses is displayed in the “Exceptions” list available in the campaign-viewing page in the “Sent” section. Note that the "Bounced” list contains the email addresses to which Mailer tried, but could not deliver, the current message (the one whose statistics you are viewing). During further campaigns, some of those “bounced” addresses will be added to the “Exceptions” list. This is how Mailer is always adding new unavailable addresses to the exception list by keeping your recipient list as clean as possible.