The lines are there to distinguish between the original messages and the replies. Its always been that way in any mail app on any device I've ever used. It's annoying if there is a long back and forth email chain, but most of the time it's barely noticable since 90% of email exchanges are message/reply and maybe a follow up. The only way to eliminate the lines would be to not quote the original message in your response. Which, of course, would make it useless for documentation.
