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 somewhat different between the new March ’08 Services Update (Mar08 SU) and prior versions. In Mar08 SU, the setting is under the System | Inbound Control tab and is labeled “Unknown Recipient Address” (set to “Allow Message”). In prior versions, the setting is located under System | Options tab and is labeled “Unknown Message Action” (set to “Forward to Internal MX”).
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.