emails being rejected

Hello

I am losing every second email from a listserver to email addresses on my cpanel managed server.

Ie.
client@unrelateddomain.com receives them everyday.
client@cpaneldomain.com receives only every second day.

I've reproduced the problem with my own email addresses.

I'm here because there is something weird about the failing emails. Here is a comparison one of the "Received From" lines in headers (linefeeds added):

This from a header that succeeds to both addresses:

This from a header that fails to one address:

Can anyone provide any advice? It is mighty strange to me that an automated list-server (mailman?) digest should have difference headers.

Needless to say, both the list owner and my hosting company both deny any problem on their end. And because I'm on a shared server with cpanel and i can't review the logs myself. My next step would be to move my customer to my VPS -- but if this fixed the problem I would then not know if other addresses on my shared server are compromised.

Thanks for any input.
Simon

 

Whatever I've been seeing in the headers has not been causing the problem. I'm not seeking a solution to my OP anymore. Thanks