Finding Forwards based upon to address

I have a distribution list that is designed to send an email to every mailbox associated with my company's domain. This distribution list only contains company generated mailboxes. When I send an email to this list, I get automatic replies from 2 email addresses that are not associated with my company. The emails state that those employees have new email addresses and to update my address book. The address that I get the reply from does not disclose the employee's name. All I know is that it's from a business position like school principal or school resource officer. Yes, I can lookup the name of the current office holder, but the account is generically tied to the position rather than a specific employee. 

 The most likely cause of this is that 2 of the email accounts in my company have forwarding turned on. How do I search my company directory to find the 2 mailboxes that are forwarding to external email addresses so I can turn off that forwarding?

I've read that this can be achieved with a powershell command but have not seen anything in the way of a working script. Nor do I have the slightest knowledge to write my own script.