To determine who has send-as permissions on a mailbox but isn’t the owner of that mailbox you can use this command:

Get-Mailbox -Identity "ltemple" | Get-ADPermission | where { ($_.ExtendedRights -like "*Send-As*") -and -not ($_.User -like "NT AUTHORITY\SELF") } | select Identity, User, ExtendedRights, IsInherited | FT -Wrap