How have you created your signature for Outlook? Did you create it within Word? Or another Text Editor? This apprears to be a simple font translation mismatch. These Greek looking letters is how the Hp 5si is interpreting the font, and is probably the only way it knows how to print it. Is it possible to re-design just the text and or fonts of your signature? You coudl use something more common such as Times New Roman within the Signature editor of Outlook. Mess around with it until it prints the way you want it to from your own machine as well as your correspondant. You don't always have to be fancy with signatures, to get the business information across to all your recepients.
Best of Luck!
Paul M. Chavez