I think upas/fs wont read messages before it's needed when
using imap. At least, I remember changing it do work that way
(Hmm, I think it required also a change in acme Mail, but I'm
not sure).

It's loooong since I used that feature for the last time, so
I'm not sure about what's upas/fs doing.

Also, IIRC, you have to read all the messages if you're using
pop instead of imap.

Once the message is here, reading it into ram is not a serious
problem unless you keep many many messages or very big ones.