By default, the Sendio ESP will reject inbound email addressed to unknown recipients. It is possible to override this setting although doing so is liable to open up a “floodgate” of spam arriving at the internal mail server.
The setting is under the System | Inbound Control tab and is labeled “Unknown Recipient Address” (set to “Allow Message”).
Ideally all email addresses are part of your internal directory which allows Sendio to enforce Recipient Checking. If you have email addresses which are not part of your internal domain it is highly recommended that they be added.