Good flexible and updated mail clients are becoming rare with the advent of 'cloud' everything...
Just come across GcMail
which has a freeware version (126.96.36.199) and a payware one (188.8.131.52), 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-installation-a-update/40-gcmail-auf-einem-usb-stick-.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...http://www.gcmail.de/en/download.html