Using domains.live.com/outlook.com to handle my mail from 1&1, how do I change the From hotmail_…@live.com on behalf

I finally have my mail set up so that my 1&1 mail is being handled by domains.live.com/outlook.com, however recipients receive my email as "From hotmail_...@live.com on behalf...".

I can't figure out how to change this.

Can anyone shed any light on this?

Regards

Tom