Some email clients seem to insist on sending html-only emails (Thunderbird, Lotus Domino, ...) - so we should probably consider supporting those too.

Here's a possible solution: jtidy+xslt->markdown
http://stackoverflow.com/questions/59557/html-to-markdown-with-java/178278#178278

http://stackoverflow.com/questions/4071370/is-there-a-decent-customisable-html-to-markdown-java-api

Tika is also capable of extracting plain text out of html very easily (arseUtils.getStringContent(part.getInputStream(), TikaConfig.getDefaultConfig(), "text/html");), but we'd loose any markup/formatting.