[dns-operations] domain has no MX record

[Stephane Bortzmeyer]

On Sat, May 07, 2016 at 06:54:50AM +0800,
 support at cloudwebdns.com <support at cloudwebdns.com> wrote 
 a message of 21 lines which said:

> when you click the subscribe button, the address is related to
> lists.ceph.com, like this one,
> ceph-users-join at lists.ceph.com
> 
> but lists.ceph.com does have no MX setup.

OK, but it has a A, so it should work (RFC 5321, "If an empty list of
MXs is returned, the address is treated as if it was associated with
an implicit MX RR, with a preference of 0, pointing to that host"). If
it does not, it means the sending mail server is broken, as simple as
that.