recida8902--- via Mailman-users writes:

When I send an email from my configured email address,

What "configured email address"? Do you mean the administrator email address for the Mailman applications? If so, that doesn't matter; the only things Mailman uses it for are (1) as the return address for some automatically generated email and (2) as a place to send certain error messages. It is not specially routed because it is mentioned in the Mailman configuration. It is specially routed because of the host it originates on, and perhaps filtered differently according to some component of the address.

it is successfully delivered to the intended recipients. However, when someone else sends an email to the same address, it does not reach the Mailman application, and no error messages are generated.

The first thing you need to do is find logs of various messages. For mail transfer agents (MTAs or "mail servers") it's usually in /var/log/postfix/mainlog (I think) or /var/log/mail.log, sometimes in /var/log/syslog or /var/log/messages. Most likely you just need to trace one example each of a message from the "configured address" to Mailman and a message from "outside" to Mailman. Find where the messages that don't get through have a receipt but no transmission.

This will most likely be from your border MTA to Mailman, or from your border MTA to your Dockerized Postfix.

3. Checked the Mailman configuration: I reviewed the configuration files of Mailman and ensured that all necessary settings are properly configured. However, I couldn't identify any specific variables or options related to incoming email handling.

There aren't any. Once it gets to Mailman, either it's related to a list or one of the sitewide administrative addresses, or it gets rejected as incorrectly addressed. Incoming mail is handled by the MTA which sorts out Mailman mail vs any other mail. All interesting configuration is in the MTA, Postfix in your case.

However, the problem is most likely at the network level, not the mail level. Configuration of Postfix for Mailman is a long-solved problem. First, your Docker host needs to be correctly configured to talk to the Internet side of your network and to the containers' network, and to function as a gateway between the two networks. The issue is that the containers use so-called private IP addresses, which cannnot be reached from the public internet.

My guess is that the most likely problem is that your border MTA is misconfigured in trying to talk to the next hop (either the Postfix MTA or Mailman itself). To figure that out, you need to trace messages until they disappear, find out what the actual IP addresses being used by each host are, and what addresses the MTAs are configured to send each mind of mail to.