That is what I ended up doing. I don't call it a fix, its just more palatable and easier to read than the way it was before. This is what I did:
I modified the line of the file:
/usr/local/frontpage/specificdomain.com:80.cnf
as
sendmailcommand:/var/qmail/bin/sendmail -F NameOfMySite -f
main-email@specificdomain.com
The result is better than it was and mail arrives now as
from: "NameOfMySite" <anonymous@admin.mainpleskserverdomain.com>
If you look in the headers or raw view of the email you also see:
Return-Path: <main-email@specificdomain.com>
From what I can tell this is the best work around possible.
I still firmly beleive, since I have seen my forms work on other Linux control panel systems
that a totaly correct solution should format the from in the email to:
"NameOfMySite" <main-email@specificdomain.com>
and not the
anonymous@admin.mainpleskserverdomain.com
Ok, I have said my peace. Hopefully someone from Plesk will find this and consider a fix in version 8.