On 23/08, René Scharfe wrote:
>Eric, hope you see this reply on the list. Direct replies to 
>sunshine@sunshineco.com are rejected by my mail provider on submit in 
>Thunderbird with the following message:
>
>    Requested action not taken: mailbox unavailable
>    invalid DNS MX or A/AAAA resource record.
>
>And with this one when using their web interface:
>
>    A message that you sent could not be delivered to one or more of
>    its recipients. This is a permanent error. The following address
>    failed:
>
>    "sunshine@sunshineco.com":
>    no valid MX hosts found
>
>It seems web.de wants you to get an AAAA record before I'm allowed to 
>send mails to you.  Sounds crazy.  Sorry about that.  Time to find a
>better provider, I guess. :-(
>

Just an A record would be enough. The issue is that mail.sunshineco.com 
has neither an A nor an AAAA record, it is a CNAME to sunshineco.com, 
which is invalid according to RFC2181.

-- 
Sincerely,
  Johannes Löthberg
  PGP Key ID: 0x50FB9B273A9D0BB5
  https://theos.kyriasis.com/~kyrias/