Even with separate records for each subdomain, I think you'd still have the problem. You'd need to do: acme MX (mail_server_ip) acme CNAME (ELB hostname) ...

Even with separate records for each subdomain, I think you'd still have the problem. You'd need to do: acme MX (mail_server_ip) acme CNAME (ELB hostname) ... but that isn't allowed. The problem is with the records conflicting, not with the wildcard.