Can virtualmin postfix by default be replaced with this artica-postfix appliance ?

Howdy -- reviewing what all that application does, I suspect it would not work well with Virtualmin.

If you wanted to use it, you could always disable the email feature in Virtualmin, and use that application entirely for managing email rather than using Virtualmin.

What I mean is that instead of using Virtualmin to create email accounts, you could tell Virtualmin to stop trying to manage email altogether, and instead fully manage email from within this other application.

Virtualmin primarily supports Postfix for email. It can also use Sendmail, though we recommend against that.

However, if you want to use a tool other than Postfix and Sendmail, you can do that, but you would need to perform all the email management from that other tool, and not Virtualmin.

To tell Virtualmin to stop trying to manage email, you can go into System Settings -> Features and Settings, and there you can disable "Mail for Domain", "SpamAssassin", "ClamAV", and "Mailman". Those are the features related to email that Virtualmin manages.