I think there is something wrong. I do have Outlook configured to view HTML mail. Since what I am finding is not the type of behavior I would expect so I did another test:

1. Send HTML email from Outlook client to both a GMail and Outlook Express clients at different domains and mail servers. Email is visible as HTML in both Outlook Express and GMail.

2. Reply to the email from both clients (Outlook Express and Gmail). Response from Outlook Express is HTML when viewed with Outlook. Response from GMail is now plain text. (I did remember to select the Send as HTML box.)

I can post the message headers for all of the test mail here if you would like.

Here's another request/suggestion: Have the Send as HTML checkbox automatically set/not set on replies and forwards based on content of original message.

Thanks for your help.
Send HTML mail