This is a daemon that handles mail processing. If you aren’t using any AV or spam filtering, I wouldn’t think it would exist…might be bug-like.
Also note that the 43M is at least partially shared libs–libs that are already loaded for other things (like Webmin and Usermin and its other processes), so actual usage could be lower than top reports (it’s actually rather difficult to figure out exactly how much RAM something requires, because the libraries it uses may be used elsewhere and thus be “free”).
But, it probably ought to be an optional item–for people that don’t receive a lot of mail and have low memory, it might be a net win to start the process only when mail arrives and let it shutdown to free up that memory again. The daemon is a relatively new invention, so the code is definitely in there to process mail without the daemon.
If you aren’t doing spam or virus filtering in Virtualmin, then this daemon isn’t needed … and in fact won’t even be setup unless you enable spam filtering on the Features and Plugins page.
To turn it off, do the following
Go to Features and Plugins and make sure spam and virus filtering are un-checked.
Go to Webmin -> System -> Bootup and Shutdown, check the box next to ‘lookup-domain’ and click the ‘Disable Now and on Boot’ button.