[Mailman-Users] Mailman only deliveres inside server

[kapuzino at web.de]

Hello

I'm using Mailman 2.1.18 as a customer of hosting.

Mailman had worked fine primarily.
After a server-update it deliveres messages only to email-adresses hostet on the same server (my own adresses connected to two different tld's).

Using my own adresses everything is going well.
External adresses only can write to the list.
They don't get any mail an any request answers.

A simple forwarding-list to the same adresses - established in plesk-panel - is doing its job.

The system is a linux debian 3.16.7.
Mailman was installed via Plesk.
It uses Postfix und Dovecot.

My hoster is a small one, I have a cheap account and I'm the only mailman user.
So I want to help him solving.

The hoster-guy is not familiar to mailman.
He changed the server two times, reinstalled mailman 2.1.5 and then 2.1.18.
Now he is out of idea.

Is this a known problem ?
A simple restriction server-setting ?

Is any further information needed ?
(phpinfo ?)

If any Error logs are required, I have to ask the hoster.

hoping for help

regards 
Kapuzino