Valid MX response includes A record for mailexchanger ?

[Kevin Darcy]

"Peter Juul S=F8rensen" wrote:

> My ISP has configured DNS-records for our mailserver. The mailserver is
> located at another ISP. Some mailservers on the Internet can mail us -
> others cannot. My guess is that the DNS-records have been configured wr=
ongly
> and some mailservers therefore can't mail us.
> When I do a MX-lookup on the ISP nameserver, that is authoritative for =
my
> domain, I get this response
> -----------------------------------------------------------------------=
-----
> -----------------------------------
> C:\T>nslookup      -type=3DMX     mail.mydomain.com.   ns.isp1.net
> Server:   ns.isp1.net
> Address: 200.200.200.201
>
> mail.mydomain.com              MX preference =3D 10, mail exchanger =3D
> mailhotel.isp2.com
> mydomain.com                      nameserver =3D ns.isp1.net
> ns.isp1.net                             internet address =3D 200.200.20=
0.201
> -----------------------------------------------------------------------=
-----
> -----------------------------------
> When I repeat this lookup a number of times, no additional records show=
 up,
> so the ISP nameserver apparently doesn't forward requests.
>
> Now, this response is not sufficient information for a mailserver to de=
liver
> mail to my mailserver.  The ip-address of my mailserver is missing, so =
a
> mailserver will need to do a second nameserver lookup for the ip-addres=
s.
> Some mailservers might not do this.
>
> My question is, must valid nameserver responses include an A-record for=
 the
> mailexchanger ?
>
> mailhotel.isp2.com                internet address =3D 100.100.100.101
> <=3D=3D=3D=3D  MUST be included ?

Responses to MX queries usually include A records for the MX targets in t=
heir
Additional Section, but even if they are missing, the sending server shou=
ld be
issuing separate queries to resolve them. Any mail server that doesn't do=
 this
is broken. It's not always *possible* to include all of those A records i=
n the
response (think truncation), so it is *required* for mail servers to fall=
 back
to issuing separate queries in at least some cases.


- Kevin