!?
You are writing about:
"....that have special name at the beginning, separated by dot from the rest of the name, e.g. aux.administracion, con.tact, nul.l etc...""
I told you that this is not the problem.
Even with a standard account like "test" it is not working.
So what does this has to do with your fix?
If this would be the main it means, that no customer is able to create e-mail accounts in the combination of P11 & ME6.75?