After sending a mailing, some members report that they did not receive the message. The log files have the following errors:
<-- DNS info available for: example.org
--> Attempting to connect to: example.org (xxx.xxx.xxx.xxx 25)
<-- Connected to: example.org (xxx.xxx.xxx.xxx 25)
<-- 421 4.7.0 Not allowed.
--> Mail stream 'default' is blocked by domain 'example.org' in step CONNECT.
In this error, example.org responds to ListManager with "421 4.7.0 Not allowed." and xxx.xxx.xxx.xxx 25 is the IP address and port of the server that responded when ListManager was trying to CONNECT to deliver the message.
The response 421 4.7.0 "Not allowed" can be caused by various factors, which are only known to the domain administrators that rejected the message. Possible reasons may include sending too high a volume of messages to the domain, the message missing some authentication protocols, or there may be an issue with the content. The recommended way to resolve this problem is to contact the domain administrators that rejected the message.
