When entering the name of the mailbox to be deleted with the correct domain prefix a red cross is shown in the retention policy editor and a notification bubble appears saying "Mailbox Not Found".
Both the domain and mailbox name appear to be valid.
The retention policy editor requires that the domain followed by the mailbox display name is entered. Domain\Username or Domain\Alias are not valid entries.
Type the mailbox display name instead of the login name, for example: