A bounce address is a type of return path in which an email address message cannot be delivered to the recipient because the email does not exist, the recipient changed hosts, the recipient has blocked the message and/or the MTA sent it to the address given by the MAIL FROM command which has many variants of the name, none of them used universally, including return path, reverse path, envelope from, envelope sender, MAIL FROM, 5321-FROM, return address, From_, Errors-to, etc.

Confused? Well to put it more simply, when you send an email to an address that no longer exists or has an issue with the server and/or email, the provider will send an email back telling you why, if it wants to. Recipient email administrators have the freedom to send a return path or not if they feel it necessary given the circumstance. Many now use invalid email addresses to set email spam traps. Soft and hard bounces are extremely important to remove from large email lists especially if the list has been sitting around for a while (inactivity).

A hard bounce is a perennial reason an email address cannot be delivered. There are several types of hard bounces. In example, the recipient email does not exist, the domain name does not exist, or the recipient email server has completely blocked the delivery. Sometimes, a recipient email server may block the message and not send a reply thus making it appear your mail is getting through. Or worse, sometimes the recipient email server may send a hard bounce message when in reality there is nothing wrong with the email, which is called a false positive.

Soft bounces typically indicate a limited (temporary) delivery problem to an address. There are several types of soft bounces. For example, an inbox may be full or the message (file) might be too large. Some mail servers set recipient emails at a quota and they have reached it. When a recipient email server is down or offline they will send a reply.

Every email marketer needs a spam trap removal service that understands the intricacies of every type of bounce in order to clean your list and identify spam traps. Many system administrators use bounces as traps and create blackhole servers to capture spam in order to build better filtering systems, so if your main mission is to remove spam traps, then bounces are the best place to look and understanding what they are, holds the key to email marketing success.