Mailman not delivering messages after updating to Exim 4.100

Symptoms

Messages sent to mailing lists are not being delivered to subscribers. When checking the mailman logs, errors like the following are found:

CONFIG_TEXT: Low level smtp error: Connection unexpectedly closed: [Errno 104] Connection reset by peer

The Exim logs will also show these errors for the connection from mailman:

CONFIG_TEXT: SMTP connection identification H=localhost A=::1 P=54902 U=mailman ID=991 S=mailman B=identify_local_connection 
H=(hostname.example.tld) [::1]:54902 F=<examplelist-bounces@exampledomain.tld> rejected RCPT <examplelist-owner@exampledomain.tld>: Authentication failed 
SMTP connection from (hostname.example.tld) [::1]:54902 D=0s closed by DROP in ACL

Cause

A conflict with a custom configuration was introduced after the Exim update. This prevents Mailman authentication from completing, and Exim terminates the connection without delivery.

Resolution

This has been identified to occur when the configuration for allow_utf8_domains is set to true. Reverting this to false in the Exim Advanced Configuration stops this error.

1. Log in to WHM as the root user.
2. Navigate to Home / Service Configuration / Exim Configuration Manager.
3. Select Advanced Editor.
4. Locate the option allow_utf8_domains and modify its text box to be false.
5. Scroll to the bottom and click the Save button.