Many potential customers are asking about departmental mailboxes in Office 365. How can a department, a team etc share a mailbox, and does it involve a license?
Shared mailboxes in Office 365 Exchange Online allow a group of users to view and send e-mail from a common mailbox.
A shared mailbox:
In Office 365 Exchange Online, shared mailboxes are created only via Remote PowerShell. See the article "A couple of tips for setting up Shared Mailboxes" - link for more details. Update March 26th 2012 - new GUI based tool for creating Shared Mailboxes
Note 1 -- From an Exchange perspective a shared mailbox is just a 5GB mailbox provisioned to a disabled user account (and delegated to a bunch of SG/Users). So when reaching the quota, mailbox will start receiving warning messages (10% prior to the quota) and then will start blocking mail sent from it (if “send as” as been granted and ‘from” used) and finally it will refuse accepted new e-mail (with “mailbox is full” in the NDR).
It is now possible to create a shared mailbox through the Office 365 web-based management portal.
I believe to open shared mailbox we need full access and not send as.
You need both full access and send as access