Throttling deliveries of email when the ISP is rejecting emails
Sometimes ISP's block messages by mistake. They may think the content is spammy, or any of a thousand different excuses.
Most Email Service Providers will remove the address from the list after 3 bounces. For discussion groups, there may be many messages sent in a day, and we don't want to remove an address for temporary problems at the ISP.
Therefore, when an ISP starts rejecting messages for a subscriber, we will start skipping deliveries, in order to give the ISP time to recover.
We maintain a database of several thousand email rejection messages that ISP's use
About half of the messages are various ways of saying this email address is not valid - that is User Unknown.
The other half of the messages are permutations of saying this message is spammy.
If we get either type of rejection message a few times, we will start skipping deliveries until the situation improves.
If the rejection is User Unknown, we will remove them from the list after 10 bounces.
If the Rejection is because of Spam, we will never remove them from the list.
The throttling starts whenever a subscriber has 2 rejections
The throttling winds down when the bounce messages stop coming back
We don't know when a delivery is 100% successful, becaue ISP's can accept the email and then bounce it back later.
We will put a notification at the top of messages after a subscriber has been throttled
They can visit the web page, where a short term archive is maintained for exactly this purpose. Messages are retained on this web page for 2 weeks.
Digest subcribers are never throttled, since we only deliver one message a day to them
List Moderators are never throttled either. And List Moderators cannot bounce off because of too many rejection messages.
So if a List Moderator address changes, you need to update the web page interface to remove them.
Example of an email address being throttled for having multiple bounces
You can reset the throttling history so that the next message will be sent
Do this if your subscriber has reported they have worked with their ISP to resolve the problem.