#Full Access

    Get-Mailbox MAILBOX | %{$foo = $_; Get-MailboxPermission $foo | ?{$_.AccessRights -eq "FullAccess" -and $_.IsInherited -eq $false}} | ft {$foo},User,AccessRights

#SendAs (this version would list all mailboxes)

    get-mailbox | %{$mailbox = $_; Get-ADPermission $mailbox.DistinguishedName | ?{$_.ExtendedRights -like "Send-As" -and $_.User -notlike "NT AUTHORITY\SELF"}} | ft {$mailbox},user,{"Send-As"}

#A version with Full Access as a CSV

    Get-Mailbox | Get-MailboxPermission | where {$_.user.tostring() -ne "NT AUTHORITY\SELF" -and $_.IsInherited -eq $false} | Select Identity,User,@{Name='Access Rights';Expression={[string]::join(', ', $_.AccessRights)}} | Export-Csv -NoTypeInformation c:\jktmp\mailboxperm.csv