Resolution
ListManager does not send rejection messages to senders when their email messages are rejected. Typically, rejection messages might be sent for one of the following two reasons:
- A sender is not a member of the list.
- A message is too large.
These rejection messages are suppressed in order to prevent Backscatter spam attacks. Backscatter can occur when List Manager sends rejection messages to forged sender addresses. As a result, malicious senders are able to send spam using ListManager as a relay.
Resolution:
It is possible to configure ListManager to allow rejection messages. Note that allowing rejection messages will cause a security vulnerability that can damage reputation and deliverability.
1. Browse to Utilities > Administration > Server > Server Settings > Security.
2. Select the Spam Blocking tab.
3. In the Error Responses section set "Suppress error response" to No.
4. Stop and restart the ListManager services.
The server will now send rejection notifications, such as the following:
Sorry your message was not sent out to [listname] because it is too large. The maximum message size is [max size] and your message was [message size] long.
NOTE: This is a software solution only. Hosted ListManager customers do not have access to this feature.
