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 sent.  

For example, you send an email to a friend that has closed their account. Their ISP will reply with an error message. We call this 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 the email by your ISP, used to transport your email to the recipient's ISP, and finally, removed by their ISP.

In the case of your personal email, the address in the From: header and the envelope sender address will be the same.....your personal email address.

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 to the server.

What happens to non-error messages?'s servers only receive error messages. All non-error messages are sent to the email sender.

Non-error messages include Vacation Auto-Responders or Challenge/Response filters (used for authentication).

What happens with email addresses that are bouncing back?

Our servers will automatically remove addresses that have bounced multiple times.

We remove bad addresses from your List so your Subscriber's ISP does not block us.

Why would an ISP block for bounces?

ISPs do not want to waste their resources by continually rejecting email for a customer that has left their service.

If an ISP blocks our IP addresses for sending to bad accounts, delivery is affected for ALL clients.

Does remove every email address that had a bounce?


Some bounces may be caused by a full mailbox or some other temporary problem. For that reason, we do not delete addresses that may start working again.

What is's bounce process?

When our server tries to deliver a message to your Subscriber's ISP, it gets 1 of 3 different types of completion codes:

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

If the completion code is a Permanent Error, it counts as 1 bounce for that Subscriber's address.

When our server receives a Temporary Error, it keeps trying every few hours. If after 3 days our server still receives a Temporary Error, it stops sending and counts that as 1 bounce.

NOTE: Some ISPs send Temporary Errors when a Subscriber's mailbox is full,
while others send Permanent Errors.

Some ISPs (like AOL) will accept email for any address in their domain ( 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 server says it has a message for [email protected], their server says "Error - no such user here."

An error message in either case counts as a bounce. After multiple bounce messages for the same Subscriber, that email address is automatically removed from your List.

Background on bounces

Traditionally, Mailing Lists would send a message with the Subscriber's address in the BCC field.  This allowed 1 message to be sent to each ISP, along with multiple addresses. This method was very efficient with bandwidth for both our servers and the ISP's. Unfortunately, it also had some downfalls:

~ The Subscriber did not see their address in the To: line, so they sometimes assumed it was Spam.
~ If one or more addresses were bad and generated a bounce, it was often difficult or impossible to
   tell which address was bad. sends each message individually. This means each Subscriber sees 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, we've noticed 3% to 5% of Announcement List addresses go bad each month. Over the course of a year, 1/3 to 1/2 of the addresses could become invalid. Some of those people may have rejoined your List using a new address. 

Reason behind bounces

Often, Subscribers simply forget to inform you of a change.

  • Some addresses go bad when your Subscriber changes ISPs.
  • A Subscriber's provider merges with another ISP and is forced to change their email address.
  • Your Subscriber has simply abandoned that particular email account.

From time to time, our server will throw away recorded bounces for email addresses with only 1 or 2. This is to prevent Subscribers that only have occasional problems from being removed from your List.

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