I am currently working on some new e-mail design templates. The web designer I am working with feels that using percentages for the table widths so that they fit to any screen size is what should be done so that the e-mail is "flexible". I always thought that fixed widths were the best practice but I cannot find documentation to state one way or the other beyond saying that e-mail should fit within 500-600 pixels (which all of this designers designs do but the headers and footers for example stretch). Which way is correct and can you show any documentation to support one way or the other?
For the content section, it does make sense to keep things fairly narrow - email reading panes are often quite small. However, many of the designs in our gallery do have headers and footers with backgrounds that stretch to 100%, and it works well.
There is no 'correct' way, but there are some designs better suited to email than others. Have a browse through the gallery and get a feel for how it might work. You can always send yourself campaigns (free to 5 or less people) to see them in the context of your email client.