Virtualmin server as MX for external domain


I’ve got (almost) successful installation of multi domain, multi IPs Virtualmin but I need to setup one of virtual servers as mail server for external domain.

I’v got few Virtualmin servers like:
domainfour.tld (setup with mail addresses I want to use, eg. john@domainfour.tld as MX)

and external server on domainfour.tld

So I set DNS MX records for domainfour.tld (on external server) to point eg. mail.domaintwo.tld

Every time I send email to address john@domainfour.tld I get error from Virtualmin server
521 No Redirect Entry for this address (in reply to RCPT TO command)
What am I doing wrong?

Do you mean that mail for domainfour.tld will be served by third party service, like G Suite? If so, then domainfour.tld has to have on its DNS zone record, MX records of that particular service.

Important: You would need to delete default MX record that is added by Virtualmin automatically or better even disable Mail for Domain feature (along with spam and virus filtering) in Edit Virtual Server/Enabled features.

No, I’m thinking about opposite situation. Virtualmin server on domainfour.tld should be mail server (something like G Suite) for external service running domainfour.tld (www is on external server and Virtualmin is for mail only).