That reason probably is good. Some mail clients wrap at 78 chars. Result is that the text doesn't flow. Larger window and text doesn't fill, smaller and you get

That reason probably is good. Some mail clients wrap at 78 chars. Result is that the text doesn't flow. Larger window and text doesn't fill, smaller and you get staggered lines. For most Outlook users, it makes sense to convert this into something more readable. I think there is an option to change its behaviour.