Outlook won't send HTML?

Suddenly my Outlook 2007 refuses to send HTML format under some
circumstances. I've seen this before on other PCs, and never found a
solution. Anybody have a clue?

 

When I send from Outlook via my Exchange account, when I have Options |
Format to "HTML" (which is my default), all emails going outside the company
are converted to plain text.

 

When I change the Options | Format to "Rich Text", it works fine.

 

When I send via my personal SMTP account, it works fine.

 

It's as if something was changed in Exchange to ban HTML, but testing on a
coworker's machine doesn't have the same problem.

 

I've restarted Outlook to no avail, but I haven't yet rebooted.

 

Any clues?