[Mailman-Users] Non-Delivery of Mail

[Barry S. Finkel]

On 1/28/2017 12:26 PM, David Andrews wrote:
> I run a cPanel system that, among other things, supports about 300
> Mailman lists. We have about 20,000 users. I am getting lots of bounced
> messages from roadrunner.com and various XXX.rr.com domains. It looks
> like they may be blocking my domain nfbnet.org although I am not sure
> how you tell.
>
> When I look at a mail delivery report via cPanel, I see messages that
> say 421 too many concurrent connections, connection refused.  What can I
> change to fix this problem!
>
> Thanks!

I have had the same problem sending to some XX.rr.com addresses,
buy not from a Mailman system.  I am sending from a system that
hosts many mailing lists; that system has 661 outbound IP addresses.
Obviously, I have no control when other organizations send e-mail,
and I have no control over what outbound address is used for the
SMTP connection.  And I do not know what the connection limits
are set by RoadRunner.  As Mark replied, I believe that this should
be a retryable error, but RR does not think so.  RR expects the sender
to re-send the mail at a later time.  I have had no problems sending
later (I have to re-compose the mail message); but again I do not know
if the re-send happens to be sent over a "good" (in RR terms) IP address
to the RR inbound mail servers.

--Barry Finkel