With a more narrow text width in your emails the eyes don't have to move as much and you seem to flow more easily through the material. It creates the illusion that there's less to read, so people will be more likely to read it.
The problem I've noticed is that the text width of my emails is different in different email programs. Some of them show my text width as perfect while in some of them my text width is too wide - way too wide. Have you noticed this yourself? This frustrates me because they're doesn't seem to be anything I can do about it. I know some of my readers will see my text as being too wide and they won't want to read it.
You also got different size screens. I checked my emails on a cellphone and the text fit the screen size perfectly. That amazed me. It just bugs me that I can't make my text size just right for everybody regardless of what email program they're using.
Is there no way to get your text width of your emails to be the same in all email programs and all screen sizes? Is it impossible for an autoresponder to format the width of your text the same in all email programs? Or is this a problem all autoresponders have?