Mailman is not delivering emails and the mailman logs report "RuntimeError: maximum recursion depth exceeded"

Symptoms

Emails being sent to a Mailman list are deliverable to Mailman but Mailman stops processing the email after acceptance.

If /usr/local/cpanel/3rdparty/mailman/logs/error contains the following error

"RuntimeError: maximum recursion depth exceeded"

and the email sent to the list are held in /usr/local/cpanel/3rdparty/mailman/qfiles/shunt/ then check the system locale on the server.

We've seen when the Turkish system locale is set python and mailman are unable to function.

First, check the system locale using

localectl status

and if it is set to tr_TR.UTF-8 it will need to be reverted back to en_US.UTF-8 so that mailman can function on that server.

This can be done by running

localectl set-locale LANG=en_US.UTF-8

and then restarting mailman with

/scripts/restartsrv_mailman --hard