kaled - 2:16 pm on Feb 11, 2010 (gmt 0)

Outlook won't let me do that.

It's a domain control panel feature, not an Outlook feature.

The reason I asked was I was wondering if Outlook might be clearing the to: header in response to an error.

I presume, since you are getting an immediate error, that you are using an smtp server on the domain name in question. If you use the smtp server provided by your ISP instead, I suspect the problem will go away.

Kaled.