The only true rule for e-mails is that they are pure text - i.e. no formatting, no layout, nowt. A bit like this answer.
Everything else is a kludge layed on top of the original standard. Which is to say, that if you create a formatted e-mail, or attach something, this is encoded into pure text (with an indicator to say that it has been encoded). How this is then decoded (or even if it is) is entirely up to the receiving client (and some user configurable options in some clients).
If you wish to send something in which the formatting is essential, your best option is to create it in a word processor, convert to pdf and send it as an attachment.