Things tunred out to be a bit more complicated than I thought (they have this tendency, haven't they?). Our customer has got two domains (A, B). Both originally pointed to their old account hosted by another provider. Their 70-something mail addresses were all of the form xyz@A .
What we did was directing domain B to to our server. After creating all mail addresses with ISPC we changed the clients's configs as well as we let the old mail boxes (by the other hoster) be redirected to our server.
Beginning at that time they fetched their mail from our mail boxes and used our server as SMTP. Their mail addresses in their clients did not change and were still of the form xyz@A . Domain A still pointed to the other hoster, but I also added A as co-domain for web B on our server.
It was just yesterday late afternoon when domain A finally got freed by the other provider and reconnected, so that it now points to our server, too.
Maybe SA got as confused as I think that you must currently be
I'm gonna change SA's config back, re-enabling the rbl stuff this evening. We'll see what happens...
If that makes things easier for you as it will surely do for me ( ; ), I may better write you an email in German...