Hi,
Ahhh... I think I understand better now.
"Mailbox Members" is a mostly display shortcut that can be used when apliccable.
To maintain more granularity the customer has to add specific rights as "Full Access" and "Send As".
The third one, which is the one they are actually asking for is "Send on Behalf Of" which is not currently available (please implement!), so I will need to add a Custom Command for that.
Thank you very much for your assistance (and patience ) with this.