I am trying to send a test campaign and it is only sending in PLAIN text, even though I've selected HTML only

I am trying to send a test campaign and it is only sending in PLAIN text, even though I've selected HTML only

I have created a draft campaign and want to test it before I send it to contacts.
Every time I send/receive the email it has no colour, graphics or layout even though I have selected HTML copy only.

Is this common? Is there a simply solution to this?