Help needed using postfix as an outbound gateway for Mailman

Folks:
I need some help diagnosing a problem using postfix as my outbound mail server with Mailman.
Up front I want to say, I have this working FINE with my current mailman list server. I'm in the process of moving my list server to an AWS lightsail instance.
And, yes, I know that I could use postfix as both the inbound & outbound mail server ... which I probably should do, but I'm far more familiar with using sendmail and have it setup & working fine on my current list server.
I've got just about everything setup and working fine.
The problem is, I'm seeing this in my mail log...
Nov 15 15:41:43 lists postfix/smtp[18136]: AA76A61E61: to=<testlist1-bounces@lists.atestdomain.net>, relay=none, delay=0.01, delays=0/0/0/0, dsn=5.4.6, status=bounced (mail for localhost loops back to myself)
It _appears_ that postfix thinks it's the mail server for lists.atestdomain.net and can't deliver the mail because it doesn't know how to deliver the message.
Here's my current setup:
I have sendmail listening on the public SMTP ports on the server. I have postfix listening on localhost:24. Mailman is configured with the SMTPHOST & SMTPPORT set to localhost & 24.
So mail is received by sendmail, sent to & processed by Mailman, and Mailman sends the mail out by connecting to postfix on localhost:24.
The list mail is being delivered fine ... but bounces are not getting processed correctly.
I have the domain lists.atestdomain.net listed in the transport map indicating that mail for that domain should be sent to localhost:25.
You can see a relatively complete log fragment at https://code.midrange.com/e665e40f93.html
Here's (what I think are) the relevant postfix config file entries ...
main.cf:
myhostname = listmail.arealdomain.com mydomain = $myhostname myorigin = $mydomain inet_interfaces = localhost mydestination = $myhostname, localhost.$mydomain, localhost transport_maps = hash:/etc/postfix/transport
transport: lists.atestdomain.net smtp:localhost:25 listmail.arealdomain.com smtp:localhost:25
master.cf 24 inet n - n - - smtpd
Any suggestions on what to look at?
david
-- IBM i on Power Systems: For when you can't afford to be out of business!
I'm riding 615 miles (Yes, you read that right) in the American Diabetes Association's Tour de Cure to raise money for diabetes research, education, advocacy, and awareness. You can make a tax-deductible donation to my ride by visiting https://gmane.diabetessucks.net.
You can see where my donations come from by visiting my interactive donation map ... https://gmane.diabetessucks.net/map (it's a geeky thing).
I may have diabetes, but diabetes doesn't have me!

On 11/15/18 8:17 AM, David Gibbs wrote:
Exactly. Postfix thinks it is the delivery agent for all mail to any of the domains in mydestination which includes localhost.
Is that all of master.cf? If there is an entry
smtp inet n - y - - smtpd
you might try removing that, or remove the 'localhost' entries from mydestination.
-- Mark Sapiro <mark@msapiro.net> The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan

On 11/15/2018 10:17 AM, David Gibbs wrote:
I decided to bite the bullet and switch to postfix for inbound & outbound mail.
david
-- IBM i on Power Systems: For when you can't afford to be out of business!
I'm riding 615 miles (Yes, you read that right) in the American Diabetes Association's Tour de Cure to raise money for diabetes research, education, advocacy, and awareness. You can make a tax-deductible donation to my ride by visiting https://gmane.diabetessucks.net.
You can see where my donations come from by visiting my interactive donation map ... https://gmane.diabetessucks.net/map (it's a geeky thing).
I may have diabetes, but diabetes doesn't have me!

On 11/15/18 8:17 AM, David Gibbs wrote:
Exactly. Postfix thinks it is the delivery agent for all mail to any of the domains in mydestination which includes localhost.
Is that all of master.cf? If there is an entry
smtp inet n - y - - smtpd
you might try removing that, or remove the 'localhost' entries from mydestination.
-- Mark Sapiro <mark@msapiro.net> The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan

On 11/15/2018 10:17 AM, David Gibbs wrote:
I decided to bite the bullet and switch to postfix for inbound & outbound mail.
david
-- IBM i on Power Systems: For when you can't afford to be out of business!
I'm riding 615 miles (Yes, you read that right) in the American Diabetes Association's Tour de Cure to raise money for diabetes research, education, advocacy, and awareness. You can make a tax-deductible donation to my ride by visiting https://gmane.diabetessucks.net.
You can see where my donations come from by visiting my interactive donation map ... https://gmane.diabetessucks.net/map (it's a geeky thing).
I may have diabetes, but diabetes doesn't have me!
participants (2)
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David Gibbs
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Mark Sapiro