Thanks for your continued help.
I am not the server-admin, so your questions are complex for me.
Meanwhile we tried to reproduce the bug, what didnt't work.
My server-admin saw in the log many attacks as reason to stop the service.
Strange temporal correlation ... but I will accept this ...
Thank you very much !!!
I must have been blind ...
As this setting is for the complete subscription, I probably didn't search at the level where settings for a single domain are made.
Now happend a second time:
After changing the subscriber of a subscription, the service "SMTP-Server (Postfix)" stopped.
Is this a known bug?
(I hope, "subscription" is the correct word for the "Abonnement" in the german Version of Plesk.)
...
Additional:
When I have a subscription with only...