What happens to mail that is undelivered to the recipient?

If a mailing is returned as undelivered (for example if the addressee is unknown at the address) then we automatically mark this inside the contacts history section as 'Returned' as shown in the screenshot below: 

We are also able to supply you with a log of all returned addressees in an Excel spreadsheet as shown in the example below where you will; see the date returned and the reason for the return along with the mailing ID: 

Please contact our Support Helpdesk to add this feature to your account.