After a very shot test of what this small email app can do. I have to say I'm very much pleased. This app is not only smaller than Thunderbird Portable, it does nearly everything else with a smaller amount of files to work with. And if you save your passwords, it encrypts thems for you!
Don't get me wrong, the Enigmail extension for Thunderbird is good. I like that it integrates with PGP/GPG. But I need something that encrypts my passwords should I choose to save them.
nPOPuk gets my vote.