You didn’t mention what the mail said (subject, etc) but if it’s one that’s being intentionally sent, then you should go into Virtualmin, select the domain, then “Edit Virtual Server” and then under “Enabled Features” make sure that “Mail for domain enabled?” is NOT checked, then save. What could be happening is that it’s supposed to be sent to “mail@domain.com” which is hosted at google, but the server thinks IT is hosting mail, so doesn’t even bother to look up the MX record. Unchecking that box should take care of it, I think.
On Edit Virtual Server, under Contact email, I haven’t specified anything here, should it be specified there do you think? So it would use an address I’ve specified instead of virtualminusername@hostname??