Flowchart of email rejection cycle:

What is a bounce?

An email "bounce" is when an error message comes back in response to the original email message sent.  For example, if you send an email to a friend and they have closed their account, their ISP will send back an error message.  We call this error message a bounce or bounce back.

Who receives the bounce?

Most ISPs follow the Internet Mail Standards, and send the bounce message back to the envelope sender address.  The email envelope is added to your email by your ISP, used to transport your email to the recipient's ISP, and then removed by their ISP. In the case of your personal email, the address in your From header and the envelope sender address will be the same.

In the case of your mailing list, the envelope sender will end with, while the From address will be your email address. This means, most ISPs will send the bounce message back to our machine.

What is your address able to receive?

Since your address is in the From header, you will get back any Vacation Auto-Responders, or Challenge/Response messages (for authentication) from your subscribers. These are not error messages, and our machine does not receive these types of messages.

What do we do with email addresses that are bouncing back?

Our machine will automatically remove addresses that have bounced multiple times. The reason we need to remove the bad addresses from your list is so your subscriber's ISP does not block us. ISPs do not want to be continually wasting their resources rejecting email for a customer that has left their service.

On the other hand, some of those bounces may have been caused by a full mailbox or some other temporary problem. For that reason, we don't want to delete addresses that may start working again.

What exactly do we do?

When our machine tries to deliver your message to your subscriber's ISP, it gets one of three different types of completion codes.

  •     Success
  •     Temporary Error - Please try again
  •     Permanent Error

If the completion code is a Permanent Error, it counts as one bounce, and we save that information.

When our machine gets a Temporary Error, it keeps trying every few hours.  If after 3 days our machine still receives a Temporary Error, it stops sending and counts that as one bounce.

Some ISPs send Temporary Errors when the subscriber's mailbox is full, while other ISPs send Permanent Errors.

Some ISPs (like AOL) will accept email for any address in their domain (i.e.  Then, should that address be bad, they will send a bounce message later (perhaps hours or even days later).

Other ISPs (like Hotmail) will error out bad email addresses before we can even send the message.  That means, when our machine says it has a message for, their machine says "Error no such user here."

An error message from either method counts as a bounce.  After multiple bounce messages for the same subscriber, that email address is automatically removed from your list.

Some background of bounces

Traditionally, mailing lists sent out a message with the subscriber's address in the BCC field.  This allowed one message to be sent to each ISP, along with multiple addresses. This method was very efficient with bandwidth.

However, the subscriber did not see his address in the To: line.  Should one or more of the addresses be bad and generate a bounce message, it was sometimes difficult or impossible to tell which address was bad. sends out each message individually.  This means each of your subscribers see only their address in the To: line. More importantly, it allows us to know exactly which address is bad when errors are generated.  While it uses up substantially more bandwidth, it saves a lot of human time.

As a general rule of thumb, I notice 3% to 5% of the addresses on an announcement list go bad each month.  Over the course of a year, 1/3 to 1/2 of the addresses on that list could become invalid.  Some of those people may have rejoined your list under their new address. 

The reason behind the bounces

Some of these addresses have gone bad when your subscriber changes ISPs, and neglects to tell your list.  Other times, their ISP merges with another ISP, and forces their customers to change email addresses. Many times, your subscriber has simply abandoned that particular email account.

From time to time, our machine will throw away the bad email addresses that have only bounced once or twice.  This is to prevent subscribers that only have occasional problems from being removed from your list.

We can adjust the number of bounces and the time period for each list. If your list has an unusual sending pattern or is losing a lot of subscribers due to bounces, please let us know.