Pat Richard posted a couple of nice one-liners to determine users who have Send-As or Full Access permissions to mailboxes. The following one-liner lists all mailboxes where another user has follow link Send-Aspermissions, and who that user is:

http://craigharline.com/books/conversions Get-Mailbox -Resultsize Unlimited | Get-ADPermission | ? {($_.ExtendedRights -like “*send-as*”) -and -not ($_.User -like “nt authorityself”)} | ft Identity, User -auto 

And this one-liner displays all mailboxes where another user has http://hilaryprince.com/canadian_mountains.html Full Accesspermissions, and who that user is:

Get-Mailbox -ResultSize Unlimited | Get-MailboxPermission | ? {($_.AccessRights -match “FullAccess”) -and -not ($_.User -like “NT AUTHORITYSELF”)} | ft Identity, User 

Both of these one-liners are very useful to determine who has access to other people’s mailboxes.