A "bounce" is a shorthand way of describing what happens when you send an email to someone and get an automatic reply from their mail server.

When an email message is delivered, special header information is added to the top of the message. This information tells a mail server or an email program who sent the message, where it came from, and where to send replies.

The FROM header identifies the sender and the REPLY TO header identifies where replies can be sent. The TO header identifies the email address of the recipient. The email address is composed of a user name and a domain name separated by the @ sign.

When an email is sent, it travels from mail server to mail server, like a hot potato, until it reaches its destination. The destination is the mail server that collects mail for the domain found in TO address. This mail server looks to see if the specified user exists internally and if so delivers the message to that user's mail box. If the user does not exist, has a full mail box, or is on vacation, the mail server will send a special notification (bounce message) back to the person in the FROM address. It is also possible to receive a bounce message from the first server the message is delivered to before it even reaches its destination. This is caused when the domain portion of the email itself is either incorrect or permanently or temporarily unavailable.

When bounce reporting is turned on, a special FROM address is constructed so that all bounce messages that arrive are collected by our software and not your email program. We then take these bounce messages and search through them to determine the cause of the bounce. This process is made difficult due to the fact that every different type and configuration of mail server returns a different type of bounce message.

Once scanned, the bounce messages are classified and uniquely identified in the bounce database. In addition to the bounced email address name, a count is kept to indicate how many times a particular email has bounced - for whatever reason.

A full bounce report can take days to assemble. If a mail server is having problems delivering a message to a domain, it may try for several days to deliver the message and then return the bounce message when it gives up.