Thank you for both the answers. I did the following for Mail User and also for Mailuser (spamfilter):
Administrator of Parallels Plesk Panel 10 may configure permissions of role Mail User so that SMB user does not see other mail accounts in Control Panel on page Mail. To accomplish that login to Parallels Plesk Panel as admin and open Control Panel of domain subscription (Subscriptions > DOMAIN.TLD subscription > Users > User Role > Mail User). In the permissions list find 'Create and manage mail accounts' and select 'Denied'.
For Mail User, in top right he can no longer see the list of domains, but he can still see "Users" tab and see all email addresses.
For Mail User (Spamfilter), he can no longer see the list of domains in top right, but he can still see "Users" tab.
Is it necessary to do BOTH the commands at
http://kb.odin.com/en/111131 or should the one quoted above be sufficient?
In any case, Sergey, can the engineers fix it so that with both Mail User and Mail User (spamassassin) they are unable to see the drop-down list of domains in the top right after making the change quoted above.
EDIT:
The mysql command hides the "Users" tab. I'm not yet sure what real disadvantage there is, if any, of removing this tab from view.
Post above is slightly wrong and corrected:
Mailuser can now no longer see the list of domains in the top right. However, mailuser (Spamfilter) CAN see all the lists of domains. Can this be fixed?