Hello,
I'm using Avast with good result. I have 3 incoming/outgoing gateways and one offsite backup mx too and Avast cost about 175USD for 5 licenses and it allows to install them on servers too (so actually if you have one mail server then you can use other 4 licenses on home or work workstations). I'm using it with Declude but it can be without any problems used as a command line scanner from SmarterMail too. Downside of SmarterMail command line scanning is that there is no way to "tell" SmarterMail that virus was found so there will be no stats for that (that's why I use Declude where I have proper logs and messages moved to virus folder). I've already sent some suggestions to SmarterMail regarding command line scanning (basically we need to have possibility to enter AV command line util return codes to let SM know if message was or wasnt infected).
Command line for AVAST:
..........\ashCmd.exe /a /c /t=A /_
Product page:
https://www.avast.com/en-us/endpoint-protection-suite
Regards