I know that there is another thread about this topic here on serverfault.com but it did not solve my issue.

I have also googled it and it seems that SPF record could be mis-configured in some cases but it did not help me neither.

Can someone tell please me why I am getting the "Reverse DNS is not a valid Hostname" error: http://mxtoolbox.com/SuperTool.aspx?action=smtp%3awebkonsulenter.dk&run=toolpage#

Thank you very much :-)

  • Strange, it works for me. Maybe MX Toolbox is having a problem. Aug 29, 2016 at 8:00
  • Same here, everything looks OK, your MX record: webkonsulenter.dk. 3600 IN MX 10 mail-hosting.dk. , the A record for MX host: mail-hosting.dk. 3598 IN A and the reverse lookup: 86400 IN PTR mail-hosting.dk.
    – HBruijn
    Aug 29, 2016 at 8:27
  • Thank you both :-) I have contacted MXToolBox Support in order to cast some light over the issue :-)
    – Jens Kirk
    Aug 29, 2016 at 13:30
  • 1
    Might it be because it's not a host name, but a domain name instead? I mean, server name should be something like mgw.mail-hosting.dk . You're using the domain name instead.
    – Mer
    Aug 29, 2016 at 13:39

1 Answer 1


Response from MXToolBox Support:

Your PTR record is mail-hosting.dk which is a Domain name. The RFC guidelines say that is must be a hostname such as mail.mail-hosting.dk. Most systems won't care that it is a domain, but since it's against the RFC guidelines, we show a warning.

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