Hi, joomleb
Yes, now I see that your task is not so simple. And I don't have an answer now.
However, you have the ability to add options for each of your sites. (If I understand the tutorial above correctly)
I haven't used it myself yet. But maybe I'll try later.
I see the main problem - how to use these options for your purposes. If you are not a programmer, perhaps specialists can help?
Stas
P.S.
Sorry, if you want the multi site settings section, you need to add options for mail (as in global configuration)? Then you need to write the developers issue