Well, that should be a receivers end decision imo. The content is there, everyone is free to view it as they please. But in essence it's plain text, no html tag shenanigans (because that's a pain to read when unparsed) or, god forbid, javascript (because it's dangerous).
Yes, that's exactly what I meant by "client". I think the tricky issue in this case relates to links because they totally ruin the text when left untranslated (in much the same way that unparsed html does), but some would prefer to view the original URL. I'd possibly advocate a different format - e.g. surround text in square brackets, supply URLs in order, at bottom of text.