Mailpile stores in RAM about 180 bytes of metadata per message (actual size depends largely on the size of various headers), but Python overhead brings that to about 250B. This means handling a million messages should consume about 250MB of RAM - not too bad if you consider how much memory your browser (or desktop e-mail client) eats up.

Totally killed my interest, I want to run this on a small server (e.g. NAS), for everyone in my family