Throttling email deliveries when the ISP is rejecting messages.

Sometimes, ISPs block messages by mistake. They may think the content is spammy.....or any of a thousand different excuses. (We've heard them all.)

Most Email Service Providers will remove the address from the Mailing List after 3 bounces. For discussion groups, there can be many messages sent in a day. That means if an ISP is having a temporary issue, several of your Subscribers could have 3 bounces in just one day.

We handle things differently at We don't want to remove an address for a temporary problem. When an ISP starts rejecting messages for a Subscriber, we begin skipping deliveries. This gives the ISP time to recover.

How does know to throttle delivery?

We maintain a database of several thousand email rejection messages that ISPs use.

We maintain a database of several thousand email rejection messages that ISP's use

Approximately 1/2 of the messages are various ways of saying "this email address is not valid" (IE User Unknown). The other 1/2 of rejection messages are permutations of saying "this message is spammy".

If we receive either type of rejection message a few times, we start skipping deliveries until the situation improves.

When does throttling begin?

Throttling starts whenever a Subscriber has 2 rejections.

The throttling starts whenever a subscriber has 2 rejections

When does throttling stop?

Throttling winds down when the bounce messages stop coming back.

We don't know when a delivery is 100% successful, because ISPs can accept the email and then bounce it later.

When are Subscribers removed for bounces?

Good question!

  • If the rejection is User Unknown, we remove the Subscriber from the List after 10 bounces.
  • If the rejection is due to Spam, we will never remove the Subscriber from the List.

How does my Subscriber know their group emails are being throttled?

We add a notification at the top of new messages. See the example below. Your Subscriber will see this message once the ISP stops rejecting email.

They can visit the web page shown, where a short-term archive is maintained for exactly this purpose.  Messages are retained on this web page for 2 weeks.

We will put a notification at the top of messages after a subscriber has been throttled

Does throttling apply to everyone?


  • Digest Subscribers are never throttled, since they only receive one summary message per day.
  • List Moderators are never throttled.  
    • Also, List Moderators will not be removed from the Mailing List due to too many rejection messages.


If a List Moderator address changes, you need to update your account and remove the old address.

Can I end throttling for a specific Subscriber?


Do this when your Subscriber reports they worked with their ISP to resolve the problem. Once you reset their throttling history, the next List message will be sent to them.

For step-by-step instructions, see Throttling: Reset for a specific Subscriber.

Example of Throttling History for an address

Example of an email address being throttled for having multiple bounces