Is it ok to make a header of my HTML email 100%? Or is it better to make the width something like 550px? I basically wanted to have a coloured bar on the top stretch to fill the users Outlook window horizontally...
100% width would probably work, but typically people design HTML emails to be quite narrow since the email reading area is often quite small.
99% would work as well and account for some discrepancies between the different email clients.
Yes, 99% will probably cause less problems overall, I agree