There are simple rules to make your email messages useful.
When replying, please reply to the top of the original message. It's much better. Although it might be easier to follow the conversation if replies are in the bottom but most of the time it's just annoying to scroll down every time.
My personal email client preference is Thunderbird. I don't recommend using web based email clients.
Well, for the 80 characters limit per line, that’s just the best width for a human to read. Plain text formatting is the best choice because it takes less bandwidth, it’s readable with any email client and it’s universal. It’s the same thing with attachments, if you’re using a proprietary format I might not be able to open it. Even if I could open it, I might not want to because of viruses. A good subject is also very important so that the email does not get eaten by the spam filter. If I ever need to find your email later on, a good subject helps me to find it.