Mailman - Bad file descriptor error when writing to mbox file

Symptoms

Some messages sent to a mailing list are not being sent to subscribed users, or stored in the mailing list's archive. In the Mailman error log, an error similar to the following is being logged multiple times:

Archive file access failure:
        /usr/local/cpanel/3rdparty/mailman/archives/private/list_domain.tld.mbox/list_domain.tld.mbox [Errno 9] Bad file descriptor

Description

When an email is sent to a mailing list, the email is processed by the "IncomingRunner" processes created by Mailman; there are 4 of these processes running at all times.  If multiple messages are sent to a mailing list in bulk, these processes will log this error when attempting to store some messages in the list's archive.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-50176. Follow this article to receive an email notification when a solution is published in the product. 

Workaround

There is currently no workaround.