I have been running inbound SmarterMail Gateway's for many years now. I have been improvement in them, but there are still a lot of issues with them. I used to run 4 inbound ones, until Greylisting delayed the emails for too long. Now I am running two, and that is still an issue, as if one gateway gets the email and greylists it, then the other one might get the email later and grelists (soft refuses it) it again because it doesn't know it already as attempted once before.
Another issue is with expired accounts and over quota accounts, it doesn't do a good job checking it again, as in the SMTP level, once it was rejected for a reason, it doesn't know about any new reasons. Example, the customer was over quota, so the email sits in the inbound server waiting and retrying, later now the account is expired and set to reject mail, it still thinks it is over quota.
I have had several tickets opened regarding issues with inbound gateways, gave feedback in old forums and beta forums, and there has been improvement made.
I was hoping the Message Sniffer feature would allow my inbound gateways to use it, however it seems all anti-spam paid services are per user bases and can only be run on the main mailserver that has the users.
One thing you can do it only allow the main server to communicate with the gateway, it won't work in my situation, as I do not use the inbound gateways for all my domains, some domains MX records point directly to the main server, so th spam checks will have to be ran, if you do not do that, you could set it that the main SmarterMail server doesn't answer on port 25 for the public but only to the inbound gateway. Thus you could disable all spam checking on the main server.
From my understanding, if you "Pass Score to SmarterMail" and the score is BIG enough to trigger a move filtering rule, it shouldn't scan it again. I thought that is how it used to work, I will need to look at my tickets and see, if I can find anything. I do see that it is scanning it again on the main server. But if it is BIG enough and you have it set to delete on the inbound server the main server shouldn't see it.
I will look everything over again, and try to update this thread with any new information. I know I am fighting spam a lot more lately with my setup, so I am sure something is not correct.