A hard bounce is an e-mail message that has been returned to you because the recipient's address is invalid (permanently undeliverable). A hard bounce might occur because the domain name doesn't exist, the recipient is unknown, the address is invalid (typos or changed address), the e-mail recipient's mail server has blocked your server, or there's some type of network problem on the recipient's end.
A soft bounce is an e-mail message that gets as far as the recipient's mail server (the address is recognized), but is bounced back before it reaches the recipient. A soft bounce might occur because the recipient's mailbox is full (over-quota), the server is down or overloaded, the message is too large or the user has abandoned the mailbox. Most e-mail service providers will attempt to deliver the e-mail regularly for a few days. If it is still undelivered, it becomes a hard bounce.
eMail Bounce Handler is a bounce e-mail filtering and handling tool that recognizes bounce e-mails using a customizable set of rules and extracts the recipients addresses allowing you to use them again to try sending your mail or to take them off your list.
eMail Bounce Handler simply connects to your POP or IMAP mailbox in order to retrieve bounces letting any other message untouched.