Due to various reasons, exchange administrator may configure the mail forwarding option on a mailbox from the “Mail flow” settings.

It’s not easy to find the forwarding later by checking the mailbox one by one. But by using the below script we can easily find any existing forwarding on all the mail boxes. 

Get-Mailbox -Filter {ForwardingAddress -ne $Null} |Select Alias, ForwardingAddress | Export-Csv -NoType -encoding "unicode" C:\Reports\MailboxesForwardTo.csv

Execute the above command from exchange power shell. It exports the list to the path provided. Above case it export to C:\Reports\MailboxesForwardTo.csv