for the smtp error its - 550 Requested action not taken: mailbox unavailable (E.g., mailbox not found, no access)
Numeric Code: 5.0.0
First Available: All numeric codes that were first available with Exchange 2000 Service Pack 1 (4.3.2, 5.4.0, 5.4.4, and 5.5.0) were classified as 5.0.0 in Exchange 2000 Service Pack 1 and earlier.
Possible Causes:• There is no route for the specified address space. For example, an SMTP connector is configured, but this address does not match.
• DNS returned an authoritative host that was not found for the domain.
• The routing group does not have a connector defined. Mail from one server in one routing group does not have a route to another routing group.
• An SMTP protocol error occurred.
Troubleshooting:1. Correct the address space, or add an address space of type SMTP with asterisk (*) value to one or more SMTP connectors.
2. Verify that DNS is working correctly.
3. Make sure that the routing groups have connectors that connect them.
4. If you are running Exchange 2000 without Service Pack 1, apply Service Pack 1 to help determine the actual issue.
for the 5.0.0 error...
ALL of these errors are RECIEVING side errors not sending side errors