let me first apologize for my english.

I work for a small local government office and have to develop a small software to manage all the mailbox we have.
We have dozen mailboxes with several users.
Mailboxes are of two types : ordinary(or corporate) and PEC(PEC is an italian acronym for a particulr mailbox legally recognised, that proofs that an email has been sent and delivered.)

Each mailbox have one (only one) owner (manager) and one or more users. A owner is also a user.
Each employe can be owner or user of many mailboxes (pec or corporate).

PEC users have their own account with personal password, but i don't have to care about it.

All users of a corporate mailbox share the same account (email address and password); the mailbox is configured locally on their mail client.
Every time a password expires, i have to change it on an intranet website and notify the new password to mailbox users (so they can change it on their client too). I have to handle this!!!

I'm developing a tool (c# winform and webbrowser control) that automatically change the password on the webstite and send the notification mail to users keeping track of date and time.

Stated that:

- for PECs i have only to keep track of their users (and owners) and if they are configured in Client mode or Web mode. Nothing more.
- for corporate mailbox I have also to keep track of the current password (not the password history), when it was modified, if it has been notified to each user and when.

This is my ER diagram (fig. 1).
Click image for larger version. 

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My questions are:
1) is it better to have the 1:N "owner" relationship or to have only the "mailbox_users" relationship with an Owner attribute? (fig2)
2) Should I map the ISA hierarchy in two different CORPORATEmailbox and PECmailbox entities? If so, i have also to "duplicate" the "owner" and the "mailbox_users" relationship (fig3)

fig.2:Click image for larger version. 

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fig.3:Click image for larger version. 

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