We have been using Smartermail Free Edition (v12.3.5318) as a Domain Forward Smartermail Incoming Gateway to Smartermail Enterprise Edition (v12.3.5318) for a couple of years. With the exception of a few minor things that are broken when running Smartermail as an Incoming Gateway (such as Mailing List bounce-backs or delivery loops when there is > 1 Incoming Gateway on a domain, etc.) it has worked fairly well.
Recently we took over using our Smartermail Incoming Gateway to do Spam Filtering for an aging mDaemon Server until we could migrate the users to Smartermail. We setup a second Domain Forward Incoming Gateway in SETTINGS > ROUTING > INCOMING GATEWAYS and everything worked the way it should...***UNTIL*** we started preparing to migrate those domains over to Smartermail Enterprise Edition. It would appear that even if an Incoming Gateway is set to "Specific Domains" in Domain Verification, this will be ignored if the other Incoming Gateway is set to Smartermail Gateway mode using "All (Web Service)" for Domain Verification.
domain.tld is defined as a "Specific Domain" for Domain Forward to 192.168.1.108
"All (Web Service)" is defined as a Domain Forward to 192.168.1.8
domain.tld (and it's users) exists on 192.168.1.108
domain.tld has been newly setup on 192.168.1.8 but no users yet migrated.
Incoming email for domain.tld will bounce because "No Such User" exists at 192.168.1.8, although it does exist at 192.168.108.
Apparently the behavior that is occurring is that Smartermail Gateway will check whether the domain exists using the "All (Web Service)" first, and if does not exist only then will it check the "Specific Domain" Domain Forward Gateway. If the domain is found using "All (Web Service)" first and the user does not exist then it will not check the "Specific Domain" Domain Forward Gateway at all.
Obviously the way to work around this would be to set the other Incoming Gateway to "Specific Domain" as well and manually enter all the domains on the primary Smartermail Enterprise server (or alternately "All but Specified Domains"). However, it seems to me that it would be more logical and economical if Smartermail either:
A.) allowed Incoming Gateways to be prioritized, or
B.) Incoming Gateways with "Specific Domains" were checked first prior to checking Incoming Gateways with "All (Web Service)" settings.
I realize that the number of installations using Smartermail as an Incoming Gateway is probably pretty small, and the number of installations using multiple Incoming Gateways is even smaller still, but it's still a problem.