Getting SMTP error from remote mail server after end of data: 550 Action not taken from Mailman lists

[RetiredAF]

We are in the process of moving to a new server to upgrade to Centos 7 and found that emails from our Mailman lists are being bounced with the reason:

SMTP error from remote mail server after end of data: 550 Action not taken​

This is occurring randomly sometimes affecting almost every member of a list and sometimes the list go out with only a few or none being bounced. There is no consistency or pattern. A member that is bounced one time might not be bounced the next time.

We used the WHM Transfer Tools to transfers the accounts. We had not had (at least for the past month) this error on the old server,

Mailman version is 2.1.33 on both the new and old servers

 

[cPanelLauren]

That error is coming from the remote server, not yours. It's possible it might be something that could be changed with the list but it's also possible that there's nothing that can be done to modify it. You'd need to provide the full error that is produced in the logs or contact the provider which is bouncing the email and identify the standards they require for an email to be sent to their servers. Since you're listed as a website owner you'd most likely need to contact your hosting provider for the full exim log data on a failed mailman list transaction.

 

[RetiredAF]

Thanks for the reply. I believe it confirms my suspicion that this is coming from somewhere outside our server. Nonetheless we don’t believe it’s coming from the remote server either. We have found a little consistency in the problem since I posted this. If we send a list we consistently see the first 5 to 7 go out ok while the remaining members of the list are bounced with the error. It seems we are not allowed to send more than about 7 emails at once. The Namecheap technical support is tell us that our emails from this list are being blocked by their “sophisticated email filtering software”. Although technical support won’t confirm this we suspect they have us connected to SpamExperts.

Some SPAM did come from our server through a forwarder. We have corrected that problem and now technical support is saying we need to wait 24 hours for our sins to be forgiven. I don’t think we’ll be any further then and I suspect this is going to take a long time to resolve. I’ll update this thread when it is, but any suggestions on how we should deal with this would be welcome.

 

[cPanelLauren]

I believe headers on the email should give you the answers you seek in terms of SpamExperts and it very well could be that or some kind of relay in place that blocks the messages and would still be considered a remote server. What I don't think at all is that the server you're on currently is the issue. I look forward to hearing what the outcome is with this.

 

[RetiredAF]

We've been able to confirm that it was, as Namecheap told us, an external “sophisticated email filtering software” which I don't believe is SpamExperts any longer. Since Namecheap won't tell us what it is I'm calling it ManBearPig as I really didn't want to believe it exists. Anyway I was able to look at the results of a blocks email on the remote server and saw

"This message was rejected at SMTP time by an RBL filter or other configuration".​

We have also determined that ManBearPig isn't reacting to spam but just to more than seven emails sent at once. Namecheap has white listed the email accounts that were immediate problems and we just recently convinced them that ManBearPIg shouldn’t be blocking legitimate (non spam) emails solely because they have more than seven recipients. They have elevated the problem.