When creating new users, if there is a system-wide alias already in existence, virtualmin doesn't notice and creates a mailbox that will never receive mail. For example, "info" is a default alias on most postfix installations.
I suspect simply noticing that an alias will prevent delivery and naming the user something else is the way to go (i.e. info-domain.tld). We could also remove the aliases during installation, but that seems intrusive.