Well, that's interesting. I had filled a bug report first, but then was informed that that would not be trivial and as such should become a brainstorm...
Anyway, were you talking about IMAP or POP3?
I would like to "merge" the two issues: yours: be able to download all and everything (configurable) to do off-line reading and replying later. mine: be able to specify in what accounts or type of accounts and how much data modest should download