Just come across GcMail which has a freeware version (3.0.0.0) and a payware one (6.0.0.6), which oddly have the same download size. The site is mostly in German but with some Google Translate massaging I found a claim that GcMail stores all of its settings in gemail.ini in the program folder (which seems to be the case) so it could be considered portable (see http://www.gcmail.de/en/support/faq/77- ... tick-.html).
While it claims to be able to synchronize its mail folders between versions/computers (good Dropbox candidate?), the interface is a terrible mishmash of English and German, with a few options disabled (clicking on them will bring up a German dialog advertising the payware version). Well, maybe this will amount to something in the future...
![Image](http://screenshots.en.sftcdn.net/en/scrn/35000/35504/gcmail-6.jpg)
http://www.gcmail.de/en/download.html