There is one more thing with incoming gateways. Try to send any email to your gateway during main smtartermail instance is starting (system is up but mailservice is still starting). Incoming gateway will bounce sent message with no such user here. I've had this problem many times during some problems with system here main smartermail instance is running.
I've sent this as a support ticket and after some reviewing I've got response:
Our developers have reviewed this issue further and at this point it is going to be considered a feature request regarding the 550 no such users.
The issue is being caused when the backup MX loses it's list of user accounts on the primary server, this is stored in memory, so if the service is ever restarted when the primary is unavailable, this issue will be encountered.
We will be adding a feature that writes this list to a flat file so that it can be queried against in the event the primary server is down. This should be included in SmarterMail 16, however I wasn't able to obtain a firm ETA. Please let me know if you have any questions or concerns.
IMHO problem is bigger for instances with higher accounts number which starts longer. After main server started I had to restart my 3 incoming gateways so they stop bouncing messages with no such user here error.