Just text-email for own domain?

when I am testing a html campaign (format: html) and I send a testmail to a mailadress which domain is the same as phplist is hosted, then I just get a text-email. If I choose another emailadress (eg gmail) then everything is fine in html.
Also the subscriber is set to HTML.
And also if I send the campaign, everybody gets in html, but the email-adresses with the hosted domain just getting text-emails.
why that?

thank you,