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There are many automatically generated reply messages that can come from sending an email marketing campaign. These automatic responses are what are commonly referred to as “bounces”. Many of these bounces contain important information about problems delivering to a particular email address. There are many possible reasons why your email marketing messages might not make it to the desired recipients inbox. For example, it is so easy for users to change to a new email account, that addresses can become undeliverable more quickly now than ever before. This articles breaks down the different responses that can result from your email marketing campaigns.

Why You Need To Keep Clean
It is very important to remove bad mail addresses from your email marketing lists. Accurate counts make for more accurate analysis of the results. Having an artificially inflated list size makes the results appear less significant than they actually are. Also, some large ISP’s are blocking email senders who try to send mailings to a significant percentage of undeliverable addresses. Finally, sending to known bad addresses wastes bandwidth resources and increases the cost to send email marketing campaigns, as there is no possible payoff from sending to bad addresses.

The Secret Life Of An Email Message
To understand email bounces we need to understand exactly how an email gets from the sender’s computer to the recipient’s. Most people who use email only interact with their email client software, such as Microsoft Outlook. The simple yet elegant communication process that takes place behind the scenes has changed surprisingly little since an engineer named Ray Tomlinson sent the first email message in 1975. What is commonly called an “email server” is really two separate pieces of software: an SMTP (Simple Mail Transport Protocol) Server, and a POP (Post Office Protocol) Server. The SMTP Server is used to send email from one email server to another. The POP Server on the other hand, is only used to retrieve email. Figure 1 below shows a simple diagram illustrating the overall process.

Figure 1

The sender’s email client software communicates with the sender’s SMTP server, which temporarily saves the message in order to forward it to the final destination. The sender’s SMTP server then attempts to connect to the recipient’s SMTP server in order to transfer the message. If for some reason it is unable to do so, the sender’s SMTP server will retry sending the message for some period of time before giving up. Once the message is transferred to the recipient’s SMTP Server, the message is saved somewhere that the POP Server will be able to find it when the recipient connects to check their mailbox. Finally, the recipient’s email client software connects to the recipient’s POP Server and retrieves the message.

Responses From The Other Side
At various points in the process, the servers used to transfer the message might send an automatic response in reply to the original message. Although different email servers can (and will) reply in radically different formats for the bounces it sends back, these responses can be separated into 10 different categories. Here are the different kinds of replies that one can get when sending email marketing campaigns:

1. Hard Bounce
A Hard Bounce indicates that an email cannot be delivered, due to a permanent error. For example, when a mailbox does not exist for a particular domain. There is no point in continuing to attempt to deliver the current message to this address, or future messages either, as the address can no longer accept email.

2. Soft Bounce
Soft Bounces are when an email cannot be delivered to an address due to a temporary error, but due to the nature of the error there is no point in re-attempting delivery of the current message. Unlike a hard bounce, a soft bounces indicates that it may be possible to deliver to the address at some point in the future. An example of a soft bounce is a “mailbox full” response.

3. Transient Bounce
A Transient Bounce is a message often generated by the sender’s email server, which indicates that a particular message could not yet be delivered, but that the server is still trying. Usually a Transient Bounce can be safely ignored.

4. General Bounce
A General Bounce is a bounce that appears to be either a Hard or Soft Bounce, but is lacking the address information of the original recipient. This usually only occurs with old or unusual email server software.

5. Blocks
A Block indicates that the recipient’s email server is refusing to accept email from the sender. This can be due to filtering software, email blacklists (lists of banned senders), or email whitelists (lists of who is allowed thru, while restricting all others). In any case the email cannot be delivered.

6. Out of Office/Auto-Reply
Out Of Office notifications are usually sent by a recipient’s email client software. Unlike bounces, an Out Of Office notification does not indicate that an email address is bad. These notifications are only really significant when sending time-dependant information to recipients.

7. Subscribe/Unsubscribe
A Subscribe or Unsubscribe message is not an automated reply. Rather, it is a request from an actual recipient to either be added or removed from mailings.

8. Virus Notifications
A Virus Notification is an email sent from a recipient’s anti-virus software that they have installed on either their client or server.

9. Address Change
An Address Change response is exactly what it sounds like: a recipient has changed their address, and is sending an automatic reply to notify senders of their new address.

10. Challenge/Response
A Challenge/Response reply is a message sent by special filtering software installed by the recipient. It is a system designed to only accept messages from known senders who have already been authenticated as being “approved”. When a sender who is not in the address book of a recipient attempts to send a message, it does not go thru right away, but instead is put into a “holding area”. An automatic reply message (the “Challenge” part) goes back to the sender to determine if an individual user or some type of mass email software was used to send the original message. If a special link in the Challenge is not clicked on (the “Response”), then the original message is not allowed on to the recipient.

Given how valuable a good in-house list can be, and how frowned upon sending to bad addresses is by administrators of the major email services, there is a tremendous incentive for a company to do their best to keep their email marketing lists accurate. The bottom line is that bounces to an email marketing campaign have to be dealt with by updating your email address database appropriately. With the potential number of bounces even a modestly sized campaign can generate, some kind of automated solution is a requirement. Fail to do a good job of bounce processing, and beware the consequences. However, handle bounces properly, and you will protect the value in your lists, and be able to utilize them to achieve your marketing objectives.

This Web Marketing article was written by Ron Evans on 3/21/2005