Exchange 2010 question

if you have the proper permissions, you can access another users Inbox or complete mailbox using the OWA 2010 GUI

Please refer below link which state how to enable send as or full access permission. if someone account is already configured then you have to remove that account

http://hosting.intermedia.net/support/kb/default.asp?id=1831

https://hosting.intermedia.net/support/kb/default.asp?id=718

By default it cannot. No user should be able to open other mailboxes by default. If that is happening then you have a problem with your permissions structure. You need to identify what permissions the user has and where they are getting them from. Only two permissions give access to mailboxes - Full Mailbox Access and Receive As. See whether the user has those, either directly or via inheritance or group membership. Then undo the permissions.

Simon.

When I add the mailbox to a new pc, the mailbox opens ok. But it opens a few other mailboxes too. I didn't set up up to begin with, would like to know how to stop it from happening.

Get-Mailbox -resultsize unlimited | Get-MailboxPermission | where { ($_.AccessRights -eq "FullAccess") -and ($_.IsInherited -eq $false) -and -not ($_.User -like "NT AUTHORITY\SELF") -and ($_.User -like "DOMAIN\YOURACCOUNT")} | ft identity, user -Wrap
change bold to your account and run command
after that remove full mailbox permissions from these mailboxes.