> ..when I deliver an e-mail address on the web I
> invent a new one that describe the site
So just create that address in sme server as a pseudonym and forward it to yourself.
> ...it is also very easy for me to disable this
> specific "unknown" e-mail address with the "e-mail > blocking" contrib panel.
Just delete the pseudonym.
Know this is possible - but it takes time! Time is not what I have most of

Also think about the number of times you have to give away an e-maill address!
> Because of the callout check this does not work
> anymore - I like to make it work again
The problem is NOT with sme server, it's a combination of how you are using the system and your ISP.
As Charlie has suggested, you need to get your ISP to stop doing that check or use my suggestion above.
I do think it is a problem with the sme server, but also a special combination of problems that cause this!
I have tried to get the ISP to change this behavior with no luck yet - I am just a very small fish in this respect!
If this is situation you can meet out there then I think SME need to handle it. I have seen it at least!
It is for sure that the feature in the server-manager panel does not work as intended when you have this callout implemented from the ISP side.