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Wouldn't a fluid approach be better? I haven't done much HTML/CSS in the last year.

I wouldn't recommend fluid layouts with email, most email clients are stuck on HTML 3.2 and have limited CSS support.

Definitely not. Short lines are easier to read. There is a reason why 72 or 80 characters are normal.

Fluid might work, depending on whether mail clients support max-width. You can cap the maximum amount of character per line like that.